Drafted into the U.S. Army near the end of World War II, Gelbart worked for the Armed Forces Radio Service in Los Angeles.

M*A*S*H earned Gelbart a Peabody Award and an Emmy for Outstanding Comedy Series and went on to considerable commercial and critical success.

On his relationship with actor Dustin Hoffman in Tootsie, Gelbart is reported to have said, "Never work with an Oscar-winner who is shorter than the statue".

His script for Rough Cut (1980), a caper film starring Burt Reynolds, Lesley-Anne Down and David Niven, was credited under the pseudonym Francis Burns.

[citation needed] Gelbart co-wrote the long-running Broadway musical farce A Funny Thing Happened on the Way to the Forum with Burt Shevelove and Stephen Sondheim in 1962.

After the show received poor reviews and box-office returns during its previews in Washington, D.C., rewrites and restaging helped; it was a smash Broadway hit and ran for 964 performances.

He also wrote the Iran-Contra satire Mastergate, as well as Sly Fox and a musical adaptation of the Preston Sturges movie Hail the Conquering Hero, whose grueling development inspired Gelbart to utter what evolved into the classic quip, "If Hitler is alive, I hope he's out of town with a musical.

He won an Emmy Award for Outstanding Comedy Series in 1974 for M*A*S*H. In 2002, Gelbart was inducted into the American Theatre Hall of Fame.