Following a period with the Manchester Library Theatre, he joined the Royal Shakespeare Company (RSC) in 1966 where he appeared next to actors such as Ben Kingsley and Ian Richardson. He made his Broadway debut as Snout in Peter Brook's legendary production of A Midsummer Night's Dream, then moved to the Royal National Theatre in the early 1980s. Over the years, Stewart took roles in many major television series without ever becoming a household name. He appeared as Lenin in Fall of Eagles; Sejanus in I, Claudius; Karla in Tinker, Tailor, Soldier, Spy and Smiley's People; Claudius in a 1980 BBC adaptation of Hamlet. He even took the romantic male lead in the BBC adaptation of Mrs Gaskell's North and South (wearing a hairpiece). He is also one of only two actors to appear in Sir Kenneth Clark's Civilisation: A Personal View series.

He also had minor roles in several films such as King Leondegrance in John Boorman's Excalibur (1981), the character Gurney Halleck in David Lynch's