Fox William Mulder was born on Friday, October 13, 1961 in Chilmark, Massachusetts (population 650). His parents were William, a retired State Department employee, and Teena Mulder. He had a younger sister named Samantha. Growing up, he enjoyed watching The Magician and playing Stratego with his sister. He was a right-fielder in baseball. He has a fear of fire after spending the night in the rubble of a friend's burned-down house. Mulder also has a photographic memory. On November 27, 1973, at 2790 Vine Street, eight-year-old Samantha was allegedly abducted by aliens. This tore the family apart. Mulder went off to school at Oxford University in England. While there, he became involved with Phoebe Greene, now a detective at Scotland Yard. In 1985, he graduated with a B.A. in psychology. In 1986, he entered Quantico Academy to become an agent with the Federal Bureau of Investigation. Upon graduating in 1988, he was assigned to the Violent Crimes Unit under Reggie Purdue. Mulder distinguished himself as an excellent profiler after his first case, pursuing a bank robber by the name of John Barnett. Barnett later pursued Mulder for sending him to jail.

In 1989, Mulder's desire to recall memories related to his sister's abduction led him to Dr. Heitz Werber, a regressive hypnotist. In 1991, he was transferred to the X-Files section, dealing with unexplained phenomena. Mulder became known as 'Spooky' Mulder because of his workings with the X-Files. Mulder was periodically under the watch of his superiors and on March 6, 1992, was assigned a partner by the name of Dana Scully.

Mulder's favorite snack food is sunflower seeds. He likes classic rock, old horror movies, and the New York Knicks. With a penchant for pornography, he subscribes to Celebrity Skin and Adult Video News. He is not married but possibly divorced.