Of his television roles, Markham is perhaps most famous for playing the dual role of Luke and Ken Carpenter in the 1967–68 ABC sitcom The Second Hundred Years, and as Harry Kellem in the original Hawaii Five-O. In 1969–70, Markham starred in Mr. Deeds Goes to Town, a television series based on the 1936 movie. Another of Markham's best known roles was as the racing-car-driver-turned-cyborg Barney Hiller in the second-season episode of The Six Million Dollar Man entitled "The Seven Million Dollar Man," which first aired November 1, 1974. Just over a year later, on November 9, 1975, in the third-season episode "The Bionic Criminal", Markham reprised the role — albeit with a character name change to Barney Hiller to not be confused with the sitcom Barney Miller which debuted in January of 1975. He appeared as Blanche Devereaux's brother Clayton Hollingsworth on the NBC sitcom The Golden Girls, and portrayed the criminally insane character Pike in the episode "Power Play" in the 1975 revival of The Invisible Man. From 1989 to 1992, he played the role of Captain Don Thorpe, senior lifeguard on Baywatch.
