Among his several appearances in the long-running Alfred Hitchcock Presents, he portrayed a gangster in high places, Mr. Koster, in the 1956 episode "The Better Bargain". In 1958, he played the villain in the first episode of NBC's Bat Masterson western series, starring Gene Barry in the title role. He appeared in four episodes of The Untouchables, including the 2 part episode, "The Unhired Assassin", as Chicago mayor Anton Cermak. In 1961, he appeared in the episode "Accidental Tourist" on the James Whitmore ABC legal drama The Law and Mr. Jones. That same year, he portrayed the highly sympathetic but fiercely dedicated state executioner in an episode of Thriller (U.S. TV series) entitled "Guillotine". He also appeared in at least one episode of Bonanza (1964). Middleton appeared as defendant "Judge Daniel Redmond" in the 1963 Perry Mason episode, "The Case of the Witless Witness". In the early 1950s, Middleton appeared on Broadway in Ondine. Other significant film roles included The Court Jester (1955) as a grim and determined knight who jousts with Danny Kaye in the famous "pellet with the poison" sequence, and as Edwin M. Stanton in The Lincoln Conspiracy (1977). In between, he played an array of brutish mountain men, corrupt cigar-biting town bosses and lynch mob Leaders. Occasionally he showed some levity, as in his recurring role as Jackie Gleason's boss on The Honeymooners (1955) sketches. Middleton guest-starred on Get Smart as the KAOS villain "The Whip", intent on hypnotizing Agent 86 in the 1970 series finale "I Am Curiously Yellow".