Mantell appeared frequently in series television, including two episodes of The Twilight Zone: "Nervous Man in a Four Dollar Room" (in a starring role) and "Steel" (co-starring with Lee Marvin). Mantell played a betrayed husband in the "Guilty Witness" episode of Alfred Hitchcock Presents. He had a recurring role from 1961 to 1962 as Ernie Briggs in six episodes of the CBS sitcom, Pete and Gladys, starring Harry Morgan and Cara Williams. Mantell also starred in season one, episode five ("Far from the Brave") of the TV series Combat!. From 1967-69 Mantell appeared five times on Mannix, four of which were in his recurring role of private detective Albie Loos.

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.

Mantell was born in the Brooklyn borough of New York City to immigrant parents from Austria. His name was originally spelled "Mantel" and accented on the first syllable, but at the beginning of his acting career, Mantell added the extra "L" and changed the pronunciation to "Man-TELL". On September 29, 2010, Mantell died in Tarzana, California, at the age of 94.