At almost exactly the same time he was cast in
Porridge (1974), he was seriously considered by producer
Barry Letts for the role of the Fourth Doctor in the BBC series
Doctor Who (1963) after
Jon Pertwee's decision to leave the series in 1974. However, he was never actually approached, as Letts eventually settled on the lesser-known
Tom Baker. By the time Baker had started his first season of
Doctor Who (1963), Mackay had already appeared on screen as his most famous character, Mr. Mackay.