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. Another of Markham's best known roles was as the racing-car-driver-turned-cyborg Barney Miller in the second-season episode of The Six Million Dollar Man entitled "The Seven Million Dollar Man," which first aired November 1, 1974. (Markham reprised the role—albeit with a character name change to Barney Hiller—in the third-season episode "The Bionic Criminal," which first aired just over a year later, on November 9, 1975). He appeared as Blanche Devereaux's brother "Clayton" on the NBC sitcom The Golden Girls, and portrayed the criminally insane character called Pike in the episode "Power Play" in the 1975 revival of The Invisible Man. From 1989-1992, he played the role of Captain Don Thorpe, senior lifeguard on Baywatch.