Composer Mark Snow got involved with The X-Files through his friendship with executive producer R.W. Goodwin. Initially when the production staff was talking about who was going to take the composing duties, Chris Carter didn't know whom to ask.
About "10–15" people were looked at, but Goodwin continued to press for Snow getting the chief composer duties. Snow auditioned around three times, but he didn't get any signs from the production staff as to whether they wanted him. Then one day, Snow's agent called him, talking about the "pilot episode", and hinting that he had got the part.
The theme music, "The X-Files", used more instrumental music score than most hour-long dramas. According to the "Behind the Truth" segment on the Season 1 DVD, Snow created the echo effect on his famous theme music by accident. He said that he had gone through several revisions, but Carter felt that something was not quite right. Carter walked out of the room and Snow put his hand and forearm on his keyboard in frustration. Snow said, "this sound was in the keyboard. And that was it."
The second episode, "Deep Throat", marked Snow's debut as solo composer for an entire episode of The X-Files. The production crew members were very careful about using too much music in the early episodes of the series.
Snow also composed the soundtrack score for The X-Files: I Want to Believe and released the soundtrack album The X-Files: I Want to Believe: Original Motion Picture Score. He recorded the score with the Hollywood Studio Symphony in May 2008 at the Newman Scoring Stage at 20th Century Fox in Century City, California.
British performers in UNKLE recorded a new version of the theme music for the end credits to the movie.Some of the unusual sounds were created by a variation of silly putty and dimes tucked between and over the strings of the piano. Mark Snow also commented that the fast percussion featured in some tracks was inspired by the track "Prospectors Quartet" from the There Will Be Blood soundtrack.