Mulder's peers dismiss his many theories on extraterrestrial or paranormal activity as those of a conspiracy theorist and supernaturalist; however, his skeptical but supportive partner, Special Agent Dana Scully, often finds them to be unexpectedly correct.

He and Scully work in the X-Files office, concerned with unsolved FBI cases that are often revealed to be supernatural or extraterrestrial in nature.

Mulder believes in extraterrestrial unidentified flying objects (UFOs) and a government conspiracy to hide or deny the truth of their existence.

[3] On 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.

[5] 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.

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.

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.

[14] After about a year in hiding, Mulder obtains crucial information from a secret facility about the date of the planned alien colonization effort, but is caught and put on trial for the murder of Knowle Rohrer.

With help from several people, including a reformed Kersh and the ghost of Alex Krycek, Mulder breaks out of prison and escapes with Scully.

[16] Seven years later, Mulder returns to the FBI when the X-Files is re-opened, in order to look into a government conspiracy, when it transpires they have been using alien technology.

Six weeks after their return to the X-Files, Mulder confronts an alive Smoking Man to try to prevent him from carrying out a plan to depopulate the United States using a virus applied to smallpox vaccines.

In a vision of Scully's, Mulder falls sick to the disease, but refuses the Smoking Man's help and his proposal to join his elite.

Despite his aloofness and cynical sense of humor, Mulder displays unbridled enthusiasm and interest when it comes to the paranormal, especially because of his personal involvement after his sister's abduction.

[9] Mulder can lose his temper when Scully is involved; on multiple occasions, he has become violent in his grief and unreserved in threatening physical force.

[19] Despite this, in the episode "Kaddish", Mulder is unable to identify a Talmudic book, states that he does not know Hebrew, and quips that Jesus returned from the dead.

[20] Also, Mulder, as his father before him,[21] had a Christian burial presided over by a Protestant minister following his apparent "death" in the episode "Deadalive".

[24] In the episode "The Field Where I Died", Mulder went into a hypnotic trance and recalls a past life, where he is in a concentration camp during the Holocaust, and another where he was a Confederate soldier killed in the Battle of Chattanooga.

The bedroom in his apartment (which appeared as late as the sixth season of the show's run) is apparently used for storage and is filled floor to ceiling with junk, including a couple of boxes of pornographic magazines.

Bill, who became disenchanted with the shadow government and his own role in the conspiracy, eventually approached Fox about his past deeds but was shot and killed by Alex Krycek – working as an assassin for the Syndicate – before he could reveal any great amount of information.

In the ninth season, "William" explains that Spender and Mulder have very similar DNA, providing strong evidence that they do have the same father.

[31][32] During his studies at Oxford, around the year of 1983, he was dating Phoebe Green; future investigator of Metropolitan Police Service (season 1, episode 12, Fire).

After the conclusion of the eighth season, Duchovny left the show and only appeared in four ninth-season episodes: "Trust No 1", "Jump the Shark", "William", and "The Truth".

At first, he wanted to base his acting career around films, but in 1993 his manager Melanie Green gave him a script of the pilot episode of The X-Files.

[37] Kolchak was in turn inspired by the fictional character Harry Picard, a policeman in Les Whitten's vampire novel The Progeny of the Adder.

[40] At the end of the seventh season Duchovny was in negotiations with the Fox network, saying he wanted more money (while not saying how much) and an improved work schedule, among other issues.

[43] Carter and most fans felt the show was at its natural endpoint with Duchovny's departure, but it was decided Mulder would be abducted at the end of the seventh season, leaving things open for the actor's return in 12 episodes the following year.

In the Millennium episode "Lamentation", the main character, Frank Black, visits the FBI Academy in Quantico, Virginia, and Mulder and Dana Scully are briefly seen descending a stairway.

Duchovny appeared as Mulder on the sketch show Saturday Night Live with Molly Shannon as Scully, when he hosted an episode.

[61] Richard Corliss from Time magazine praised Duchovny for settling in his role so "quickly" and calling the character "an obsessive plodder".