Also a steady fixture on television, he appeared in such made-for-TV films as adaptations of A.J. Cronin's Beyond This Place (1957) and The Citadel (1960), Bonanza (1961), and Brenda Starr (1976). He also played the title role in a Philco Television Playhouse version of Othello and acted in a CBS production of Beyond This Place (1957). He appeared in programmes such as The Real McCoys, Thriller, Gunsmoke, Perry Mason, Voyage to the Bottom of the Sea, Daniel Boone and Mission: Impossible. He was cast as the sly space trader in an episode of Lost in Space. Thatcher, if not for his distinguished career, has become recognised by several generations of fans of Star Trek: The Original Series from his performance as Marplon, secretly a member of the resistance in the episode "The Return of the Archons".