|Spoilers for the Mass Effect series follow.|
Prehistoric Times (Unknown BCE - 1,000,000,000 BCE)
- The Leviathans dominate the galaxy, enthralling every new sapient race they encounter and protecting them in return for tribute. The Leviathans consider themselves the galaxy's first and only apex race.
- After observing that their subservient races create synthetic races who consistently and violently rebel, the Leviathans create an Intelligence to seek a way to permanently preserve organic life. The Intelligence constructs an army of "pawns" to gather genetic information throughout the galaxy.
- The Intelligence betrays the Leviathans, using its pawns to slaughter them for their genetic material to create Harbinger, the first Reaper. Thus begins the cyclical harvest of the galaxy's intelligent organic life by the Reapers, part of a vast experiment conducted by the Intelligence to fulfill its mandate.
- The Intelligence builds the mass relay network to increase the efficiency of the cycles, allowing galactic civilizations to develop faster and more consistently between harvests.
Pre-Prothean Times (1,000,000,000 BCE - 68,000 BCE)
- The Leviathan of Dis, a Reaper, is killed by the Leviathans. Its corpse comes to rest on the planet Jartar and remains undisturbed for nearly one billion years.
- An unknown spacefaring race fires a mass accelerator round at a Reaper near the planet Mnemosyne. The round penetrates the Reaper, disabling it, and continues moving through space, eventually striking the planet Klendagon and creating the geological feature there known as the Great Rift Valley.
- The ancient arthenn race flourish in the Zelene system, living on the planet Helyme until being destroyed in an unknown event that wiped out all complex life on the planet. They also maintained a presence on other planets in the system including Epho, which bears the scars of an ancient orbital bombardment, and Gaelon, which may have been mined for helium-3.
- Ancient spacefaring races called the thoi'han and inusannon fight over the planet Eingana, littering the planet with the debris of hundreds of starships. Refined element zero scattered by broken drive cores contaminates the environment, causing many native species to go extinct and those that survive to show a tendency to develop biotic powers.
Pre-Council Times (68,000 BCE - 500 BCE)
- The Protheans achieve spaceflight and discover mass effect technology from the ruins of the extinct inusannon. They go on to establish a galaxy-wide civilization linked by the mass relay network with the Citadel as their capital.
- At some point, the Protheans encounter a hostile race of machine intelligences that endanger their existence. To combat this threat, the Protheans aggressively assimilate many other spacefaring organic races into their empire. The Prothean Empire is able to fend off the machines in a conflict known as the "Metacon War".
48,000 BCE: Fall of the Protheans
- The Prothean Empire collapses with the arrival of the Reapers through the Citadel. Although the Protheans had learned of the Reapers' existence from studying the ruins of previous civilizations and had begun to prepare, they were nonetheless caught off-guard. Over the next several centuries, the Protheans fight the Reapers system by system, world by world, and city by city. In the end, it is not enough. The Reapers methodically wipe out the remaining Protheans, as well as other contemporary races, and depart the galaxy to await the next cycle.
- Seeking to escape the imminent explosion of their sun, an alien race constructs an AI-managed starship equipped with supercomputers containing a virtual world, into which some one billion of the aliens transfer their consciousnesses. The starship begins a journey throughout the galaxy which lasts for the next 8,000 years.
- Tuchanka, the krogan homeworld, enters the nuclear age. In a global conflict, weapons of mass destruction are released, triggering a nuclear winter. In the resulting devastation, krogan society devolves into a collection of warring clans.
- A supernova propels the Mu Relay, the only point of access to the remote Pangaea Expanse, out of position. Concealed somewhere in the dense nebula formed by the supernova, the relay's position is effectively lost for centuries. Later, the rachni rediscover the relay.
- After developing faster-than-light spacefaring capabilities based upon Prothean technology, the asari begin to explore the mass relay network, and eventually discover the huge Citadel space station at a hub of many mass relays.
- The salarians discover the Citadel and open diplomatic relations with the asari.
Council Era - Formation (500 BCE - 1 CE)
500 BCE: Founding of the Citadel Council
- The Citadel Council is formed. The asari and salarians together colonize the Citadel and establish it as a center of the galactic community, led by the Council. This year is also known as 0 GS, the beginning of the Galactic Standard (GS) timeline.
- As a gesture of openness with their new asari allies, the Salarian Union opens the records of the League of One. Under threat, the League responds by assassinating every member of the Union's inner cabinet; Special Tasks Group operatives then hunt down and eliminate the League.
- First contact is made with the volus. Eventually, the Citadel Council commissions the volus to draw up the Unified Banking Act, which establishes a standard galactic currency known as the credit and links all galactic economies.
- The turian Unification War occurs. The increasingly isolated and xenophobic colonies on the frontiers of turian space go to war with each other. After years of fighting, the Turian Hierarchy sweeps in and pacifies the remaining factions. Animosity between turian colonies continues for decades.
- The Council grants the volus the honor of being the first non-Council species with an embassy at the Citadel, rather than a Council seat.
- First contact is made with the batarians. They are granted an embassy a century later.
- The asari discover the elcor home system and help the elcor locate and activate their nearest mass relay. "Within one elcor lifetime" they establish a regular trade route to the Citadel and are granted an embassy.
Council Era - War and Rebellion (1 CE - 900 CE)
1 CE: The Rachni Wars
- The rachni, a species of highly intelligent hive-minded insects, are discovered when a Citadel expedition opens a dormant mass relay leading to their star systems. The rachni prove to be hostile and begin a war with the rest of the galaxy. Negotiation with the rachni queens is impossible because they cannot be contacted in their underground nests on the toxic rachni worlds.
- The Rachni Wars continue. The salarians make first contact with and uplift the primitive krogan, manipulating them into acting as soldiers for the Citadel Council. The krogan prove able to survive the harsh environments of the rachni worlds and pursue the rachni into their nests, systematically eradicating queens and eggs.
- The rachni are declared extinct. In gratitude for their aid during the Rachni Wars, the Council rewards the krogan a new homeworld. Free of the harsh environment of Tuchanka, the krogan population explodes.
300 - 700 CE
- The krogan begin to expand exponentially, colonizing many new worlds. Growing concerns about their expansion lead to the founding of the Special Tactics and Reconnaissance branch of the Citadel.
- Beelo Gurji, a salarian operative, is appointed the first Spectre by the Citadel Council.
700 CE: The Krogan Rebellions
- Krogan warlords leverage veterans of the Rachni Wars to annex territory from other races in Citadel space. Eventually the Council demands withdrawal from the asari colony of Lusia, but the krogan refuse. A preemptive strike is made on krogan infrastructures by the Spectres. The Krogan Rebellions begin.
- The Citadel Council makes first contact with the turians around this time and persuades them to aid in the war. After the krogan respond to the initial turian offensive by devastating turian colonies with weapons of mass destruction, the turians vow to stop the krogan from ever becoming a threat again.
- Sometime after the turians join the galactic community, the volus are accepted as a client race of the Turian Hierarchy.
- Realizing that the krogan will never give in as long as they can replenish their fighters, the turians unleash a salarian-engineered bio-weapon known as the genophage on the krogan. The krogan population starts its decline.
- The Krogan Rebellions end, though scattered krogan insurgent actions continue for decades. The turians fill the military and peacekeeping niche left by the decimated krogan.
- The Citadel Conventions are drawn up in the wake of the conflict.
Council Era - Expansion (900 CE - 2157 CE)
- The turians are granted a seat on the Citadel Council in recognition for their service in the Krogan Rebellions.
- Extensive and unchecked industrial expansion on the drell homeworld Rakhana begins taking a significant toll on the planet's environment.
- The first sightings of the Collectors are reported in the Terminus Systems. In Citadel space, the claims are dismissed as unsubstantiated rumors and tall tales.
- (Estimated date) In the Andromeda galaxy, the Jardaan begin deploying Remnant machines to the Heleus Cluster to terraform barren planets into life-supporting worlds.
- Samara's three daughters are diagnosed as Ardat-Yakshi. Rila and Falere accept exile, but Morinth flees. Samara takes the oaths of the asari justicars and pursues her errant daughter for the next 430 years.
- Aria T'Loak arrives on Omega, a space station in the Terminus Systems regarded as the region's nominal capital. Aria is hired by the station's warlord as an enforcer and quickly becomes one of his most trusted lieutenants.
1895 CE: The Geth War
- The geth, machines created by the quarians as a source of cheap labor, become self-aware. Fearing a geth uprising, the quarians begin dismantling them. The geth revolt against their quarian masters. In the resulting conflict--known to the geth as the Morning War--the geth systematically drive the quarians from their own worlds. The surviving quarians are reduced to living as spacefaring nomads aboard the Migrant Fleet. Contrary to expectations, the geth do not venture outside the former quarian systems into wider Citadel space, instead isolating themselves from the rest of the galaxy behind the Perseus Veil. As punishment for creating the geth, the Citadel Council closes the quarian embassy on the Citadel.
- The geth begin construction of a mega-structure designed to house and simultaneously run every geth program in existence. Completion of the mega-structure, which would allow the geth to maximize their collective processing capacity, is the long-term goal of geth civilization.
- April 12: Yuri Gagarin aboard the Vostok 1 becomes the first human to travel in space. Humanity becomes a spacefaring race.
- July 20: Apollo 11 lands on Luna. Neil Armstrong becomes the first human to walk on Luna, and the first human to walk on an astronomical object other than Earth.
- Aria T'Loak assumes control of Omega after defeating the space station's previous warlord in single combat. She mockingly bestows the title "Patriarch" on the broken krogan, and uses him as a reminder to her enemies of her power.
- Sojourner touches down at Ares Vallis becoming the first rover to explore Mars. During its three months of operation, the rover captures more than 550 images of the Red Planet.
- The drell make first contact with the hanar around this time. With their homeworld Rakhana severely depleted and no spaceflight capability, the drell were poised for a massive population crash by 2025 CE. Agreeing to help, the hanar mount a large-scale rescue operation and evacuate approximately 375,000 drell to their own homeworld, Kahje, over the following decade. The remaining 11 billion drell on Rakhana gradually perish, warring over the last reserves of food and water.
- SpaceX successfully lands and recovers the first stage of its Falcon 9 rocket after delivering 11 communications satellites to orbit. This represents the first-ever orbital class rocket landing, and humanity’s first major shift towards sustainable space travel.
- July 20: Armstrong Outpost at Shackleton Crater is formally founded as the first human settlement on Luna, on the 100th anniversary of the first lunar landing.
- Billionaire Victor Manswell, frustrated with the pace of official human space exploration, begins funding his own private spaceflight expedition.
- The Manswell Expedition successfully launches from Earth en route to the Alpha Centauri system with 300 colonists aboard in cryogenic stasis. Communications with the vessel are lost soon after.
- Liara T'Soni is born.
- The European Space Agency's Lowell City in Eos Chasma becomes the first permanent human settlement on Mars.
- The yahg, an intelligent pre-spaceflight race, are discovered on the planet Parnack. Ambassadors representing the Citadel Council arrive on the planet and attempt to open diplomatic contact. After the ambassadors are killed by the predatory yahg, the Council ceases all contact with the yahg and declares Parnack off-limits.
- Not long after the yahg are discovered, one yahg is captured for the Shadow Broker to study. With the Broker's assistance, the yahg rapidly ascends within the Broker's organization as "Operative Kechlu". Eventually, the Broker becomes wary of "Operative Kechlu" and attempts to have him terminated, but the order is sent too late. The yahg assumes the mantle of the Shadow Broker and directs the organization for the next sixty years.
- Alec Ryder is born.
- Steven Hackett is born.
- David Anderson is born.
- The Eldfell-Ashland Energy Corporation demonstrates helium-3 fuel extraction from the atmosphere of Saturn.
- Saren Arterius is born.
- Kahlee Sanders is born.
- Thane Krios is born.
2148 CE: Humanity Discovers Mass Effect Physics
- Humanity discovers a small cache of highly advanced Prothean technology hidden deep beneath the surface of Mars on the south polar region of Promethei Planum. Building on the remnants of this long extinct race, humans quickly explore the science of mass effect fields, leading to the development of faster than light travel and beginning detailed exploration of the Sol system.
- Following information from the translated data cache on Mars, humans discover that Charon, Pluto's moon, is actually a massive piece of dormant Prothean technology, a mass relay, encased in ice. Once activated, Jon Grissom leads the first team of explorers through the relay, which instantaneously transports them to another relay in Arcturus, 36 light-years away. The explorers discover that the mass relays are part of a vast network, making travel across the galaxy possible.
- The Systems Alliance charter is signed by the eighteen largest nations on Earth. The Alliance soon becomes the military and exploratory spearhead of humanity.
- Miranda Lawson is born.
- The Systems Alliance begins the first surveys for colonization prospects outside the Sol System. One of these surveys discovers the planet Terra Nova.
- To defend its expanding territory, humanity begins constructing a massive military fleet and space station at Arcturus, the nexus of several key mass relays, even though they have yet to encounter another intelligent spacefaring race.
- An accident at Singapore International Spaceport exposes hundreds of humans to dust-form element zero. Roughly 30% of the children born in Singapore after element zero exposure suffer from cancerous growths. After his mother is exposed during the accident, Kaidan Alenko is born later that year with biotic potential.
- The Delta Pavonis Foundation, a major consortium based on Earth, begins settlement of humanity's first extra-solar colony world, the planet Demeter. Later that year, additional colonies are founded on Eden Prime and Terra Nova.
- April 11: Shepard is born.
- There is a second publicized accident involving the exposure of humans to element zero.
- The Systems Alliance occupies completed portions of Arcturus Station, intended to become the Alliance's headquarters.
- Jeff "Joker" Moreau is born.
- Arcturus Station is formally inaugurated.
- A small number of human children exposed to element zero exhibit minor telekinetic abilities.
- Kai Leng is born.
- Pluto's orbit becomes circularized as a result of mass relay operations.
- Jacob Taylor is born.
- David Anderson graduates from Officer Candidate School with the rank of Second Lieutenant.
Council Era - Advent of Humanity (2157 CE - 2183 CE)
2157 CE: The First Contact War
- Humanity makes violent first contact with another spacefaring race: the turians. The turians observe human explorers attempting to activate a dormant mass relay, a practice forbidden by galactic law after the Rachni Wars, and attack. Over the next three months, a brief but tense conflict known by humans as the First Contact War and by turians as the Relay 314 Incident ensues.
- The events of Mass Effect: Revelation begin. Admiral Jon Grissom travels to Arcturus Station to congratulate graduates of the Systems Alliance's N7 training program and meets David Anderson. Grissom announces that an unknown alien force has attacked an Alliance post at the colony of Shanxi.
- The First Contact War culminates in the turian siege and occupation of Shanxi, the first human world to fall to an alien race. Admiral Kastanie Drescher leads the Second Fleet in battle against Shanxi's occupiers one month later, catching the turians by surprise and evicting them from the planet.
- The turians prepare for a full-scale war against humanity, but this draws the attention of the Citadel Council. The Council intervenes before hostilities escalate further, revealing the existence of the greater galactic community to humanity and brokering a peace between them and the turians.
- The events of Mass Effect: Evolution occur. While aiding the human resistance effort on turian-occupied Shanxi, mercenary Jack Harper becomes embroiled in a plot by turian general Desolas Arterius to use an ancient alien artifact, the Arca Monolith, to convert the turian race into an army of invincible "meta-turians". Harper convinces Desolas' brother Saren of the dangers of the Monolith, which is "devolving" the turians into mindless beings under the Monolith's control. To protect his people, Saren orders the destruction of the building holding the Monolith, Temple Palaven, with Desolas and the meta-turians inside. Later, having realized that humanity and the galaxy face dark times ahead, Harper founds the human-survivalist organization Cerberus, publishes its manifesto, and assumes the identity of the Illusive Man.
- An anonymous extranet manifesto is published following the end of the First Contact War. The manifesto calls for an army, a "Cerberus", to be established to defend humanity against inevitable alien attacks. Derided as "survivalist rhetoric written by an illusive man", the manifesto is quickly forgotten by the media.
- Humans learn the potential of biotics. An international effort to track element zero exposures begins. Roughly 10% of recorded exposed children show indications of biotic ability.
- April 14: Ashley Williams is born.
- The Systems Alliance purchases Gagarin Station for a fraction of its construction cost, and converts the facility into a biotic research and training facility.
- The Systems Alliance Parliament is formed.
- With the existence of human biotics firmly established, the Biotic Acclimation and Temperance Training (BAaT) program is set up on Gagarin Station to train potential candidates and develop biotic implants.
- The biotic drug red sand is first used.
- Kasumi Goto is born.
- Amid concerns over gene therapy and genetic modifications being misused, the Systems Alliance Parliament passes the Sudham-Wolcott Genetic Heritage Act. It imposes sharp restrictions on controversial uses of genetic engineering, but provides government subsidies for beneficial applications.
- Tali'Zorah nar Rayya is born.
- Jack is born.
- Construction of Arcturus Station is completed.
- A survey team working for batarian entrepreneur Edan Had'dah discovers a mysterious artifact orbiting an unnamed planet near the Perseus Veil.
- The Alliance secretly begins illegal AI research at a base on Sidon with Dr. Shu Qian as project leader. Lt. Kahlee Sanders is assigned as a tech analyst.
- A series of starship drive failures are orchestrated over populated areas on human colony worlds, causing widespread exposures to element zero. This results in a second generation of humans born with biotic potential.
- The Leviathan of Dis, a genetically-engineered living starship believed to be nearly a billion years old, is discovered by a batarian survey team on the planet Jartar. The Leviathan disappears under dubious circumstances after a batarian dreadnought passes through the Dis system.
- Alec Ryder's twin children are born.
- Ivor Johnstagg is sentenced to 21 years in prison for attempting to assassinate Venta Tox, the volus ambassador to the Citadel. Although Johnstagg claims to be acting on behalf of the radical human political party Eternal Earth, no connection is found and Johnstagg is diagnosed with paranoid schizophrenia. Backlash from the assassination attempt jeopardizes the Systems Alliance's bid for an embassy on the Citadel.
- Humanity continues to expand, founding more colonies and establishing trade alliances with many of the other species who recognize the authority of the Citadel Council. In 2165 CE, the Council makes official recognition of humanity's growing power and influence in the galactic community. Humanity is granted an embassy on the Citadel, the political and economic heart of the galaxy. Tensions grow between humans and batarians as they compete for territory in the Skyllian Verge.
- The events of Mass Effect: Revelation continue. After an Alliance research station at Sidon is attacked, David Anderson searches for those responsible alongside Kahlee Sanders, and discovers an Alliance scientist, Dr. Shu Qian, was conducting illegal AI research to unlock the secrets behind a mysterious artifact discovered near the Perseus Veil. Eventually, Anderson is assigned to work with Saren Arterius to track down Dr. Qian and his batarian supporter, Edan Had'dah. However, Saren instead kills both Dr. Qian and Had'dah, blames Anderson for the mission's failure, and uses Dr. Qian's research to search for the artifact himself.
- Terrorists steal antimatter from the Alliance cruiser SSV Geneva. The sole figure arrested names his sponsor "Cerberus". This is the first such incident of sabotage connected to the Cerberus organization.
- After a disagreement between Vido Santiago and Zaeed Massani over hiring batarians into the Blue Suns, Vido betrays Zaeed and leaves him for dead. All records of Zaeed's involvement in the Blue Suns are erased.
- Jack is abducted by Cerberus agents on Eden Prime. They fake her death and take her to the Teltin Facility on Pragia as part of a project intended to improve biotic potential in humans.
- L2 biotic implants are first developed and used in humans. Kaidan Alenko is among the first to receive the new implants.
- The magazine Fornax is launched.
- Kaidan Alenko accidentally kills Commander Vyrnnus after being provoked. The diplomatic fallout with the turians results in BAaT being shut down a year later.
- Shepard receives secondary exposure to element zero. Permanent biotic inclination manifests.
- Another human colony, Yandoa, suffers catastrophic dust-form element zero exposure to its atmosphere when an Eldfell-Ashland Energy ship explodes in orbit. Many children suffer birth defects; thirty-seven biotic children are born, including Gillian Grayson.
- Biotic training for humans is outsourced to the military, selected R&D companies, and renamed divisions of the defunct Conatix Industries.
- L3 biotic implants are developed after L2 implants prove to be dangerous.
- The first A-61 Mantis Gunships are produced.
- In protest at the refusal of the Citadel Council to limit human expansion in the Skyllian Verge, the batarians close their embassy, withdraw into their home systems, and effectively become a rogue state.
- Shepard is officially detected as a biotic and fitted with L3 implants.
- Cerberus assassinates Pope Clement XVI on Earth via rosary beads coated with toxic substances; his death is attributed to age and heart failure. His successor, Pope Leo XIV, espouses beliefs more in-line with Cerberus' ideals.
- A fly-by over the world of Armeni discovers odd surface protrusions. Upon closer inspection, they are identified as elaborate crypts made by the zeioph, an extinct spacefaring race. Various human universities seek to perform excavation, but Council laws that hold burial sites as sacrosanct prevent this. An ongoing debate over the issue ensues.
- Terra Firma is involved in a Nashan Stellar Dynamics kickbacks scandal, forcing Inez Simmons, the current party leader, to resign. Charles Saracino becomes the new party leader after his opponent, Claude Menneau, is assassinated by Cerberus operatives while en route to Shanxi. Publicly, Menneau's disappearance is never explained.
- On a survey mission to 2175 Aeia, an important colonization prospect for the Systems Alliance, the MSV Hugo Gernsback disappears and is presumed destroyed.
- Cerberus covertly contacts Michael Moser Lang, a political dissident on Earth, and provides him with funding to purchase weapons. Cerberus continues to monitor him after contact ends. One year later, Lang assassinates United North American States president Enrique Aguilar and Chinese People's Federation premier Ying Xiong. The resulting political shuffle benefits Cerberus' shell companies and plans for the Systems Alliance Parliament.
- Batarian-funded pirates and criminals launch a surprise attack, later known as the Skyllian Blitz, on the human colony of Elysium. The assault is repulsed by the Alliance Navy and ground teams.
- The Jon Grissom Academy is commissioned over Elysium, and becomes home to the Alliance's new biotic training program, the Ascension Project.
- The Anhur Rebellions, a civil war waged over the practice of slavery, begin on the human and batarian colony world of Anhur and spread throughout the Amun system. Abolitionism supporters hire the Eclipse mercenary group to fight the batarian pro-slavery Na'hesit faction.
- Kai Leng is awarded the N7 designation by the Alliance's Interplanetary Combatives Academy, but later that year he is dishonorably discharged and imprisoned for first-degree murder.
- Miners at the T-GES Mineral Works facility on Mahavid uncover a Leviathan artifact. The miners are enthralled by the Leviathans for the next ten years and are used to carry out numerous experiments.
- The Andromeda Initiative, a privately-funded multi-species endeavor created to explore and colonize the Andromeda Galaxy, is founded.
- During a recon mission to find a missing colonial pioneer team on Akuze, a unit of fifty Alliance marines is wiped out by thresher maws.
- In retaliation for the Skyllian Blitz, the Alliance launches a major offensive against the moon of Torfan and destroys the criminal bases there, mostly populated by batarians. The threat against human colonies from batarian extremists is curtailed.
- The Alliance tracks several pirate FTL exit vectors over a period of six months using covert monitoring devices planted on Theshaca's moons, leading the Alliance Navy to eight major pirate anchorages. Since the "Theshaca Raids", no ships from the Terminus Systems have entered the Hong system.
- A lethal airborne virus sweeps through the quarian Migrant Fleet. Tali'Zorah nar Rayya's mother is among the casualties.
- The Anhur Rebellions end with the abolitionist forces in power, and the reconstruction of Anhur commences.
- Admiral Kahoku of the Systems Alliance begins investigating suspected Cerberus activities.
- Humans and turians collaborate on an engineering project co-sponsored by the Citadel Council: an experimental frigate with a prototype stealth system, the SSV Normandy. David Anderson is given command of the ship, and Commander Shepard is assigned as executive officer.
- The events of Mass Effect: He Who Laughs Best occur. Jeff "Joker" Moreau is assigned as the pilot of the SSV Normandy.
Mass Effect (2183 CE - 2186 CE)
2183 CE: The Eden Prime War
- The events of Mass Effect occur. The human colony of Eden Prime is attacked by the geth, initiating a wider conflict between humans and geth known as the Eden Prime War. After exposing the involvement of rogue Spectre Saren Arterius in the attack, Commander Shepard and the crew of the SSV Normandy pursue Saren in his search for the Conduit, later revealed to be part of a larger plan orchestrated by the Reaper Sovereign to return its kind to the galaxy. While Shepard follows Saren through the legendary Mu Relay to the Conduit, Sovereign and a geth fleet assault the Citadel. Shepard defeats Saren and the Alliance Navy destroys Sovereign, preventing the release of the Reapers. The Citadel Council is irrevocably changed by Shepard's decisions.
- The events of Mass Effect Galaxy occur. Jacob Taylor and Miranda Lawson prevent an assassination attempt on the Citadel Council by batarian terrorists.
- The events of Mass Effect: Incursion occur. Aria T'Loak interrupts a meeting between Blue Suns mercenaries and the Collectors on Omega, and learns that the Collectors have a dangerous interest in humanity.
- The events of Mass Effect 2 begin. One month after the Battle of the Citadel, the SSV Normandy is attacked and destroyed by an unidentified assailant, resulting in the death of Commander Shepard. Later, the Systems Alliance officially declares Shepard "killed in action".
- L4 biotic implants are developed.
- The events of Mass Effect: Ascension occur. Two months after the Battle of the Citadel, galactic society is still in flux over the changes to the Council and kept unaware of the true nature of Sovereign, the Citadel, and the mass relays. Cerberus pushes ahead with plans for both the Ascension Project and the quarian Migrant Fleet, but is foiled by the actions of Kahlee Sanders and Paul Grayson. After an attack by Cerberus forces, the Migrant Fleet changes its conservative policy, sending ships away from the Flotilla to look for new homeworlds or possibly a dormant Reaper.
- The events of Mass Effect: Redemption occur. Commander Shepard's body is recovered by Blue Suns mercenaries for the Shadow Broker, who has been hired by the Collectors. Cerberus eventually obtains the body with the help of Liara T'Soni and begins work on the Lazarus Project.
- The events of Mass Effect: Paragon Lost begin. The Systems Alliance dispatches several special forces units to aid the colony of Fehl Prime, which is under attack by Blood Pack forces. Most units are shot down before landing, but Delta Squad survives and defeats the Blood Pack. The Alliance assigns Delta Squad to guard the colony.
- The Eden Prime War comes to a close. Although most geth forces in Citadel space were destroyed in the Battle of the Citadel, holdouts yet remain. The Alliance Navy reduces its patrols, relying instead on civilian ships to report any geth activity. Raids are carried out against identified geth outposts, but the conflict is essentially over.
- The raloi of the planet Turvess launch their first space telescope and discover the asari cruiser Azedes in their system. The asari make first contact with the raloi, who are formally welcomed into the galactic community the following year.
- The biological weapon EHE, or "exotic humanoid encephalopathy", is used by the human terrorist organization Totenkopf in an attack on Gagarin Station. The Alliance cruiser SSV Manila is deployed to monitor the asteroid Israfil, the supposed origin of the miroorganisms used to produce EHE.
- The freighter MSV Estevanico is attacked by Blood Pack mercenaries and crashes into the planet Zanethu. The Alliance crew on board were among the first humans to encounter the vorcha race.
- A clone of Commander Shepard, developed by Cerberus for testing purposes alongside the original's under reconstruction body, is stolen by the deserter Rasa from the Lazarus Research Station.
- L5 biotic implants are developed.
- July 25: The Andromeda Initiative orientation briefing is shown to prospective candidates.
- The events of Mass Effect: Paragon Lost continue. Fehl Prime is attacked by the Collectors after a Cerberus agent hoping to discover their intentions signals them. Most of Delta Squad is lost during the battle for the colony, but they succeed in disabling the Collector ship. Delta Squad member James Vega chooses to rescue the asari Treeya, who possessed vital intel on the Collectors, leaving the colonists to die aboard the Collector ship when it crashes on the planet's surface.
- The events of Mass Effect 2 continue. Commander Shepard is revived and tasked by the Illusive Man to investigate the mysterious disappearances of entire human colonies in the Terminus Systems. Investigation of the attacked colony of Freedom's Progress reveals the enigmatic Collectors from beyond the Omega 4 Relay to be responsible for the disappearances. After assembling a capable team, Commander Shepard uses a captured Reaper IFF to safely traverse the relay and infiltrate the Collector base. Shepard succeeds in eliminating the Collector threat, saving humanity throughout the galaxy from certain destruction.
- February 10: A dangerously close flyby of the comet CR1331 Kingu devastates the hanar colony of Belan.
- March 28: A coalition of the corporations Binary Helix, Sonax Industries, and Guanghui Solutions invades the krogan and vorcha-populated world of Garvug.
- April 28: The Vallum Blast annihilates much of the capital city of the turian colony of Taetrus. In retaliation, Taetrian colonial and Turian Hierarchy troops declare war on the separatist group responsible for the attack.
- August 11: The Citadel Council makes first contact with a race of virtual aliens who offer advanced technology in exchange for help maintaining the supercomputers that run their civilization.
- Liara T'Soni locates the Shadow Broker's concealed base on the planet Hagalaz. The Shadow Broker, exposed as a yahg, is killed. Liara takes control of the Broker's organization, intending to use the Broker's vast information network to aid Shepard's fight against the Reapers.
- December 7: Biologists discover the kirik, an insect-like species with biotic capabilities, on the arid world of Ekram. After the kirik exhibit signs of intelligence, experts and officials debate whether or not the kirik should be uplifted.
- The Andromeda Initiative begins its journey to the Andromeda galaxy, with the first wave, made up of the Nexus, Ark Hyperion, Ark Leusinia, Ark Natanus, and Ark Paarchero, launching into dark space. A second wave comprised of Ark Keelah Si'yah is expected to launch not long after.
- January 12: An asari exploration team discovers a lost human colony in the Alpha Centauri system. The Systems Alliance links the colony to the forgotten Manswell Expedition of 2070 and establishes contact with the colonists.
- January 23: SSV Budapest, an Alliance cruiser that sparked a diplomatic crisis between the Earth Systems Alliance and the Batarian Hegemony, withdraws from the Bahak system.
- The Bahak system is subsequently destroyed following the explosion of its mass relay. An asteroid was sent to the relay in order to delay the Reapers' invasion. Hundreds of thousands of batarians living on Aratoht are obliterated in the blast.
- The events of Mass Effect: Retribution occur. Six months prior to the start of the story, Systems Alliance hero Jon Grissom dies. Hundreds of dignitaries attend Grissom's funeral and a memorial plaque dedicated to him is placed in the Jon Grissom Academy.
- After three years on the run, Paul Grayson is captured by Cerberus and implanted with Reaper technology. Kahlee Sanders and David Anderson attempt to rescue Grayson by persuading the Turian Hierarchy to raid several vital Cerberus installations, but are unaware Grayson's body is now under Reaper control. Once free, the Reapers learn of the Ascension Project and use Grayson to single-handedly attack Grissom Academy, where he is finally killed by Anderson and Cerberus assassin Kai Leng. In the aftermath, Anderson and Sanders decide to study Grayson's body to discover how to combat the Reapers.
- The events of Mass Effect: Inquisition occur. Councilor Udina asks Captain Bailey to investigate Executor Pallin, claiming that C-Sec has been compromised and is working against the Citadel Council. The investigation results in Pallin's death and Bailey's promotion to the rank of Commander, but Bailey is left with reservations about Pallin's guilt.
- The events of Mass Effect: Deception occur. Gillian Grayson seeks revenge against Cerberus and the Illusive Man for the death of her father. With the help of a faction of biotic supremacists, she captures Cerberus assassin Kai Leng to lure the Illusive Man out of hiding by holding Leng for ransom. Her plan fails, however, and Leng kills her and escapes.
- The events of Mass Effect: Conviction occur. Admiral Anderson travels to Omega to recruit Alliance marine James Vega to guard Commander Shepard as the Commander is taken to trial for the destruction of the Bahak system.
- The events of Mass Effect: Invasion occur. Aria T'Loak defends Omega from Adjutants unleashed by Cerberus. The Adjutants distract Aria from Cerberus's true plan: to conquer Omega and maintain exclusive access to the Omega 4 Relay. Aria realizes this and unites the disparate gangs and mercenaries of the station to repel the Cerberus invaders, but is outsmarted by Cerberus's General Oleg Petrovsky. When he threatens to destroy Omega, Aria accepts exile and Cerberus takes control of the station.
- The events of Mass Effect: Infiltrator occur. When a Cerberus alien experimentation facility known as the Barn is attacked by turian forces, Randall Ezno, a Cerberus operative enhanced with biotic implants, learns the truth about the horrific nature of the experiments and discovers his own handler has been made a test subject. Randal turns against Cerberus and the Barn's Director, fights his way through the facility, and escapes. He provides the Systems Alliance with intel on his former employers and swears revenge on the Director.
- September 21: A massive influx of batarian ships in the Exodus Cluster reported by the Alliance News Network. The newcomers claim their home systems are under attack from an as-yet unidentified hostile adversary. Khar'shan is reported to be under bombardment and the Harsa relay blockaded.
- September 28: Taetrus goes dark, the culprits confirmed to be the same forces behind the attack on the batarians. Primarch Fedorian declares the Turian Hierarchy at war.
- The events of Mass Effect 3 begin. Six months after the destruction of the Alpha Relay, the Reapers invade the galaxy through batarian space and attack Earth. Commander Shepard escapes and discovers a Prothean superweapon design on Mars that could end the Reapers' threat once and for all.
- October 5: Hierarchy offensive to retake Taetrus reported as failed: four turian fleets dispatched to the Mactare relay, Reaper ships emerge from the Trebia relay in a push toward Palaven. Primarch Fedorian alerts the galaxy to prepare for spotty communications due to destruction of comm buoys during wartime.
- The events of Mass Effect 3 continue. To build the Crucible and take back Earth, Commander Shepard and allies embark on a journey to unite the various races in the midst of a galaxy-wide war. During the final confrontation with the Reapers over Earth, the Commander makes a decision that changes the galaxy forever.
Andromeda Bound (2186 CE - 2818 CE)
- (Estimated date) The Scourge is unleashed in the Heleus Cluster, attacking all Remnant technology it can find. Remnant vaults are deactivated, the golden worlds decline, and angaran civilization falls into a dark age.
- (Estimated date) The Scourge stabilizes, and angaran civilization recovers. Five angaran worlds, including Havarl and Voeld, rediscover spaceflight and reunite, finding they have developed divergent cultures. The angara discover and colonize Aya and Kadara.
- The kett invade the Heleus Cluster of the Andromeda galaxy. While the main goal of the invasion is to exalt the angara into the Kett Empire, the invasion force's Archon becomes obsessed with learning how to control Remnant technology.
- Evfra de Tershaav reorganizes the angara's many ineffective resistance movements against the kett into the Angaran Resistance, based out of Aya.
New Frontiers (2818 CE - 2819 CE)
2818 CE: Touchdown Andromeda
- The events of Mass Effect: Nexus Uprising occur. The Nexus arrives in the Heleus Cluster of the Andromeda galaxy after a 633 year journey through dark space.
- Ark Leusinia, Ark Natanus, and Ark Paarchero arrive in the Heleus Cluster separately. They are either attacked by the kett or damaged by the Scourge, preventing them from rendezvousing with the Nexus.