Before Season 3
Though Lauren revealed early in the season that she was trained as a CIA field agent at The Farm, her father, believed the occupation too dangerous and used his influence to make sure she did no evaluation. Thus, she became an agent of the NSC and served as liaison to the CIA.
Lauren and Sydney had an antagonistic relationship from the start, with Lauren in particular feeling threatened that her husband's former lover was back in his life. Eventually, Sydney and Lauren started co-operating and even went on missions together. During one mission, Lauren shot and killed a man, an act which apparently left her distraught.
However, it was soon revealed that Lauren was actually a double agent for The Covenant, working to sabotage the CIA's work against the criminal organization. She often used Vaughn to gain more information about Sydney. She was also revealed to be a cold-blooded killer. Her first known mission for the Covenant was the assassination of Andrian Lazarey.
She was tasked to murder her own father when he was given evidence of her true affiliations as well as to fix her faltering relationship with Vaughn. When Lauren could not bring herself to shoot her father, her mother (also a Covenant agent) stepped in and pulled the trigger herself. Lauren was an adulterer, not only did she seduce some of her targets, she also became involved (personally and professionally) with Julian Sark, who was also working for The Covenant.
Eventually, her true nature was discovered by the CIA and she became a marked woman with both Sydney and Vaughn wanting to kill her. This did not stop her from infiltrating the CIA office disguised as Sydney; she shot Marshall Flinkman and caused major damage to the building. She injured and killed several other agents with remote-triggered bombs before she murdered an innocent motorist and stole his car to escape.
During a vicious hand-to-hand combat, Sydney barely lost out to Lauren, but the latter was shot by Vaughn before she could kill Sydney. Before her death, she revealed to Sydney the location of a safe deposit box containing vital information about Sydney's family. This information turned out to be evidence that the CIA authorized Jack Bristow to kill his wife and Sydney's mother, Irina Derevko; an order he apparently had carried out (Resurrection).
The CIA determined that Lauren's death should not be made public and her remains were housed in a special high-security mortuary. Lauren's corpse was shown to Sark, who demanded to see her body (and then declared his love for her in front of Vaughn). (A Man Of His Word).