Mantell appeared in the episode "Storm Center" of the Westinghouse Desilu Playhouse, as well as the pilot of the crime drama, The Untouchables, starring Robert Stack that originally aired as a 2 part episode of the WDP. He later turned up in The Untouchables series itself, in the 2 part episode, "The Unhired Assassin", where he played Giuseppe Zangara, the would-be Assassin of Franklin Delano Roosevelt who ended up accidentally assassinating Chicago Mayor Anton Cermak instead. In the 1974 film Chinatown, Mantell played Lawrence Walsh, associate of private eye Jake Gittes. He delivered the film's famous last line, "Forget it, Jake, it's Chinatown." The character of Walsh reappeared in The Two Jakes. He had a small role in Alfred Hitchcock's The Birds.