Johnny Mandel composed incidental music used throughout the film. Also heard on the soundtrack are Japanese vocal renditions of such songs as "Tokyo Shoe Shine Boy", "My Blue Heaven","Happy Days Are Here Again", "Chattanooga Choo Choo", and "Hi-Lili, Hi-Lo"; impromptu performances of "Onward, Christian Soldiers", "When the Lights Go On Again", and "Hail to the Chief" by cast members; and the instrumental "Washington Post March" during the climactic football game. Columbia Records issued a soundtrack album for the film in 1970.
MASH features the song "Suicide Is Painless", with music by Mandel and lyrics by Mike Altman, the director's then 14-year-old son. The version heard under the opening credits was sung by uncredited session vocalists John Bahler, Tom Bahler, Ron Hicklin, and Ian Freebairn-Smith (on the single release, the song is attributed to "The Mash"); the song is reprised later in the film by Pvt. Seidman (played by Ken Prymus) in a scene in which a character, a dentist called "Painless", attempts suicide. Altman has noted in interviews that his son made quite a bit more money off publishing royalties for the song than the $70,000 or so he was paid to direct the film.