When she was six years old Sydney Bristow's mother Laura was killed in a car accident. Throughout her childhood her father Jack was absent, spending most of his time on work. During her freshman year at college a man approached her saying that the U.S. government wanted to talk to her about a possible job. Feeling lonely and out of place she called them. She was accepted to SD-6, a covert division of the C.I.A. and after six months she tested to be a field agent. She excelled and was told that she was a natural. Sydney worked as a field agent for seven years, finishing college and entering graduate school at the same time. During her time at SD-6, run by Arvin Sloane, Sydney went on various missions around the world believing that she was protecting her country. As seven years passed Sydney's friends believed she worked at a bank called Credit Dauphine since one of the main rules at SD-6 was that you do not tell anyone you work in the intelligence field.
Sydney dated Daniel Hecht, who was studying to be a pediatric cardiologist for two years when he proposed. Sydney happily accepted but the fact that she was lying to Danny about her job weighed heavily on her shoulders. She finally decided to tell him everything. Though Danny had a hard time accepting it at first, he realized that the most important thing was that he loved Sydney. However, Arvin Sloane discovered that Sydney had told him and ordered Danny's execution. Sydney returned from a mission in Taipei to find Danny's bloody body in his bathtub.
Sydney left work for three months, which left SD-6 questioning her loyalty. She was attacked one night in a parking garage and was shocked when it was her father who came to her rescue. Jack Bristow revealed to his daughter that he worked for SD-6. In addition he told her that SD-6 was not a black ops division of the C.I.A. as she had been led to believe but a branch of the Alliance of Twelve, a group of agents who went freelance about a decade before and dealt in arms, among other things. In her father's words, she was working for the very people she thought she was fighting.
With this new information Sydney went to Taipei on a mission to regain Sloane's trust. Once this was completed she walked directly to the Los Angeles offices of the Central Intelligence Agency and began her life as a double agent. In this position Sydney met with her handler, Michael Vaughn, to discuss counter missions that would thwart the plans of SD-6. She also discovered that her father was also a double agent and the two who barely knew each other were now depending on each other for their lives.
As time went on at SD-6, Sydney's relationship with her father improved and she and Vaughn began to develop strong romantic feelings for one another. Through a series of events Sydney discovered that her mother was not dead but was a Russian spy named Irina Derevko who married her father to gather intel from the United States. SD-6 began to pursue a man named Alexander Khasinau who, as it turns out, was Irina's K.G.B. superior.
Also going on at SD-6 was the search for artifacts from Milo Rambaldi, a man believed to be a prophet who lived 500 years earlier. Rambaldi had told a prophecy that a woman in a picture he had drawn would use his works to bring about great suffering, a woman referred to as The Chosen One. The woman was Sydney, who Sloane was having collect Rambaldi artifacts from around the world in his own obsessive pursuit for Rambaldi's endgame.
During this time Sydney's friend Will Tippin began to investigate Danny's death which led him to the name SD-6. His investigation went further and Jack was forced to step in before Sloane ordered Will's execution. Jack and Will worked together and Will finally found out about Sydney's double life.
On another mission in Taipei with Vaughn, Sydney was captured and found herself face to face with Khasinau. She assumed he was the man in charge but soon learned differently when her own mother appeared, gun in hand. When Sydney wouldn't tell her mother the information she wanted, Irina shot her daughter. Sydney escaped and not long after Irina turned herself in to C.I.A. custody.
Sydney had a newfound confidante in Will, whose career as a reporter ended so that he could keep his life. At the same time the two had to deal with lying to their friend Francie Calfo, though they knew it was for her protection.
While in C.I.A. custody Irina gave valuable intel that aided the United States, though Jack was still skeptical of his wife. Sydney began to form a bond with her mother, who seemed to genuinely care for her.
The C.I.A. soon received information that not only helped them in taking down SD-6, but the entire Alliance. However, Arvin Sloane escaped and it was revealed he was behind the Alliance's destruction. Sydney could now be seen in public with Vaughn and the two began their relationship. Her SD-6 partner Dixon and OpTech Marshall Flinkman joined her at the real C.I.A.. Little did Sydney know that her friend Francie had been killed and a woman who looked identical had taken her place. The woman worked for Arvin Sloane.
Irina Derevko went on a mission with Jack and at that time she escaped with the help of Arvin Sloane. Sydney felt betrayed by her mother, who told her via Morse code transmitted through a pair of earrings that "truth takes time."
Irina was tracked to Mexico after Sloane had Jack kidnapped. She and Sydney had one last meeting on top of a building where Irina told her daughter she loved her before disappearing. Sloane, meanwhile, had pieced together his Rambaldi artifacts, waiting for a message to be revealed to him.
Back in Los Angeles Will, who was dating Francie now, discovered that his girlfriend had been doubled and was really a woman named Allison Doren. She and Will fought and Will was stabbed and left for dead, but not before leaving a message regarding his findings for Sydney. Upon hearing the message she and Allison engaged in a fight that ended with Sydney shooting Allison and Syd herself being knocked unconscious. When she woke up she found herself in an alley in Hong Kong with a mysterious scar on her stomach.
Sydney called the C.I.A. and informed them of where she was and was told to go to the safehouse. She waited there until Vaughn came in, looking visibly shaken. She noticed that he was wearing a wedding band and he told her that he was indeed married and no longer with the C.I.A. and that Sydney had been believed to be dead since her apartment burnt down nearly two years earlier.
In her time gone Sydney learned that Dixon had been made head of the Los Angeles offices of the C.I.A., that Sloane had become a humanitarian who has been pardoned by the U.S. and now helped them and that her father had not believed her to be dead and his search for her landed him in prison. She worked to get him out and returned to work to find that Vaughn's wife, Lauren Reed, was working with the N.S.A. and currently investigating the murder of Russian diplomat Andrian Lazarey. Jack, who has been working with Irina, showed Sydney a video he'd uncovered that revealed to him that his daughter was indeed alive. The video showed a blonde Sydney slitting the throat of Lazarey.
Lauren's investigation led her to Sydney who was then taken into custody and tortured in order to get information. Jack, Vaughn and Sloane worked to get Sydney free and were soon aided by Lauren. Sydney went to a doctor with an experimental procedure to try and regain her memory. In her dream state she saw Will, who survived and was now in witness protection. She found him and the two went for information regarding her missing years. She discovered that Lazarey was alive and knew her by the name of Julia Thorne. She also faced Allison Doren, who had survived and Will killed her.
Sydney was taken from her home by F.B.I. director Kendall, who she had originally met when the Rambaldi prophecy was revealed and when he had served as director of the Los Angeles C.I.A. office. He had information about the past two years for her. Sydney had been taken by a group known as The Covenant and that was searching for Rambaldi artifacts. They had discovered that Rambaldi left behind some of his D.N.A. and with the eggs from The Chosen One to make a baby, explaining the scar on Sydney's stomach. The Covenant kept her captive, trying to brainwash her into believing she was Julia Thorne. In order to get free from them she acted as though she was Julia Thorne and immediately went to Kendall. She tried to resume her normal life but saw Vaughn with Lauren and went into hiding, helping Kendall and also helping Lazarey search for the Rambaldi D.N.A. to keep it from The Covenant. They hid it and in order to get The Covenant's suspicion away from them they faked Lazarey's murder and Sydney had her memories erased so that she could not tell the location of the sample should she be tortured.
The mystery of her two missing years solved, Sydney was involved in the C.I.A.'s efforts to take down The Convenant. They discovered that there was a mole in the agency and worked to find it. Unbeknownst to them the mole was Vaughn's wife, Lauren. Working with The Covenant she gave them intel that allowed them to be a step ahead of the U.S. government. Vaughn refused to believe that accusations against his wife but soon had to face them as well as his feelings for Sydney.
Before he was assassinated Lazarey told Sydney about The Passenger. She soon learned that The Passenger was not a thing, but a person. A result of an affair between Arvin Sloane and Irina Derevko produced The Passenger, Sydney's sister. Sydney located the woman, named Nadia Santos, but Sloane got a hold of her. The Passenger was meant to be able to communicate directly with Rambaldi and deliver his message. Sloane used his daughter to get to this but she was rescued by Sydney.
Lauren was revealed to all as the mole and went into hiding. She and Sydney soon faced off and before Lauren was killed she told Sydney that nothing in her life is a chance, that it was all planned out and there are no coincidences. She gave Sydney the number to a safe deposit box in Wittenberg. Sydney opened it and found her name on documents for a C.I.A. sanctioned project headed by her father that began on April 17th, 1975 - the day Sydney was born. The subject of the project was Sydney herself.