The son of Broadway legend Zero Mostel, whom he somewhat resembles, Josh Mostel began in show business as a boy soprano at the Metropolitan Opera in New York. He had already made his professional stage debut in 1968 before graduating from Brandeis University. Graduation brought on-going employment in films and on stage, with Mostel nonplussed by accepting a juicy one-scene part or being a member of an ensemble cast. He made his Broadway debut in 1971 in "Unlikely Heroes", and his film debut that same year in "Going Home". In the 1979 sitcom "Delta House" (ABC), Mostel took the part of Bluto Blutarsky, the role originated by John Belushi in the feature film "Animal House". It lasted nary a season, and his subsequent series efforts were equally short-lived: the military sitcom "At Ease" (ABC, 1983) and the detective series "Murphy's Law" (NBC, 1988-89).

Mostel has had somewhat better luck on the big screen. He was a bouncing King Herod in Norman Jewison's "Jesus Christ Superstar" (1973), was seen in Alan J Pakula's "Sophie's Choice" (1982) and played the bumbling brother of a murderer in "Compromising Positions" (1985). His biggest roles have been as Uncle Abe, sharing the house with the young Woody Allen alter ego and his family in "Radio Days" (1987), and as Barry Shalowitz, the ice cream magnate who knows that given his looks he had better hire models to sell his ice cream in "City Slickers" (1991). Mostel also did a cameo as Shalowitz, again with David Paymer as his brother, in "City Slickers II: The Legend of Curly's Gold" (1994) and played a high school principal in the Adam Sandler vehicle "Billy Madison" (1995). In 1984, Mostel began an association with John Sayles when he played Casio Vendor in "The Brother From Another Planet", co-produced by Mostel wife's Peggy Rajski. He went on to appear in other Sayles-directed films: in "Matewan" (1987), he was the mayor of a town caught up in a coal miner's strike, while in "City of Hope" (1991), he was the owner of a discount appliance store.