Middlemass played the philandering butcher Mr. Lyon in the final episode of the third season of the British series Upstairs, Downstairs in 1973, but it was not until 1980, when he appeared in the post-World War I drama To Serve Them All My Days, that he first took a leading role in a British series. He followed this up with a notable performance as The Fool to Michael Hordern's King Lear, and also played Brezhnev in Tom Stoppard's Squaring the Circle. He went on to play minor characters in Yes Minister, Winston Churchill: The Wilderness Years (as Lord Derby), Juliet Bravo, Only When I Laugh, All in Good Faith, Yes, Prime Minister, Oliver Twist (as Mr. Brownlow), Sherlock Holmes and the Leading Lady, and Miss Marple, in the 1989 episode "A Caribbean Mystery" (as Major Palgrave). From 1992 to 1993, he appeared in twenty episodes of the police drama Heartbeat as Dr. Alex Ferrenby. Following that in 1993, Middlemass first appeared in the sitcom As Time Goes By as Rocky Hardcastle, a role that continued regularly until 2002, and then also in the 2005 two-part reunion specials, his final television appearance. He has appeared in both British TV adaptations of The Adventures of Sherlock Holmes, "The Blue Carbuncle" playing Peterson in the BBC adaptation and Henry Baker on the Granada TV series.