The son of Meyer C. Ellenstein, a Newark dentist, Robert Ellenstein grew up in that New Jersey city and saw his father go on to become its two-term mayor. He served in the United States Army Air Forces during World War II: earning a Purple Heart during his Service. He attended New York University and graduated with honors from the University of Iowa. He began acting, directing and teaching in Cleveland, Ohio. A veteran of the "Golden Age" of live television (he played Quasimodo in a live Robert Montgomery Presents (1950) version of "The Hunchback of Notre Dame"), for the same show played the lead in "A Case of Identity", later turned into the film The Wrong Man, he was the first actor to play Albert Einstein on television. Ellenstein made his first film in 1954 (MGM's Rogue Cop), he was featured in Alfred Hitchcock's North by North West. In 1961, he played the mobster Legs Diamond in an episode of NBC's 1920s crime drama The Lawless Years with James Gregory.