Mulder was born on October 13, 1961, on the island of Martha's Vineyard, Massachusetts. The mysterious disappearance of Mulder's sister and his ensuing search for her became the consuming drive of his life. In 1983, Mulder graduated with first-class honors from the University of Oxford with a Bachelor of Arts degree in psychology. He then graduated with honors from the Quantico FBI Training Academy in 1986. Mulder joined the FBI on October 24, 1984. Upon graduating from the academy, Mulder began his work in the Behavioral Science Unit (psychological profiling) under Special Agent Bill Patterson, with whom he had a testy relationship.
In 1988, the FBI assigned Mulder to the Violent Crimes Unit. Around this time, Mulder first came across the X-Files, an obscure FBI section dealing with cases relating to the paranormal; he pored over these cases in his spare time, becoming obsessed with them. In 1991, Mulder re-opened the X-Files with Special Agent Diana Fowley, but Fowley left shortly thereafter. Mulder investigated the X-Files by himself until March 1992, when Special Agent Dana Scully, an instructor at the FBI Academy's Forensic Science Research and Training Center, was partnered with him for the purpose of applying scientific reasoning to Mulder's work and theories.
Mulder's ultimate goal was to uncover what he believed to be a government conspiracy to hide the truth about alien life, and to find out what had happened to his sister. His quest and belief in the existence of aliens were, for most of the series, the driving force of the plot, or mytharc. However, during a period of time after Scully's cancer went into remission (season 5), Mulder was convinced by Michael Kritschgau that aliens did not in fact exist and that the government conspirators had merely concocted that threat as a smokescreen, to justify military activities and toy with him.
During the seventh season, Mulder eventually discovers the truth about his sister. Samantha was abducted (either by the alien colonists or the government conspirators), and various tests were performed on her. She was then returned to The Smoking Man to live out her life under his supervision, all the while undergoing additional tests. She was unable to bear the testing any longer so she ran away from her home and was eventually admitted to a nearby hospital, where she disappeared from her locked room. It is revealed that Samantha was taken by "spiritual intervention", with the help of beings called "Walk-ins". Mulder is briefly reunited with Samantha's spirit.
Mulder was also abducted by the aliens himself in 2000, and returned to Earth, almost dead, a few months later. He had been infected by an alien virus (for the second time), but Scully found a way to rescue him. After Mulder returned home to his apartment, an application was submitted to the FBI for his reinstatement to the X-Files but this was declined by Deputy Director Alvin Kersh, who had been promoted in charge of the X-Files shortly after Mulder had been abducted. He returned to work for a brief period of time, but was eventually fired for failure to follow orders not to investigate any X-Files. After Scully gave birth to William, Mulder went into hiding in New Mexico after Kersh said his life was in danger.