Stewart had begun his stage career with such repertory companies as the Liverpool Playhouse and the Playhouse Theater in Sheffield. In 1961, he began a tour of Australia, New Zealand and South America with Vivien Leigh and the acting company of the Old Vic. Stewart's career really began to shift into high gear in mid-decade: he made his London debut in 1966, joined the RSC in 1967, and made his Broadway debut as Snout in Peter Brook's landmark production of "A Midsummer Night's Dream" in 1971. Work on television picked up soon afterwards; Stewart etched an especially memorable portrait of villainy as Sejanus on the acclaimed BBC series, "I, Claudius" (1976).

For years, Stewart's film roles were generally smaller and less colorful than his stage work, with the exception of his manic romantic Eilert Lovborg in "Hedda" (1975), an adaptation of Ibsen's "Hedda Gabler" in which he supported Glenda Jackson. Beginning to appear in US films in the early 80s, Stewart did his professional best in roles that foreshadowed his later fame in fantasy and science fiction. He played a small part in John Boorman's Arthurian fantasy "Excalibur" (1981) and appeared as one of the Duke's advisors and a fighting tutor to Kyle MacLachlan in the muddled but intriguing adaptation of Frank Herbert's "Dune" (1985). Stewart's fame from the "Star Trek" series changed all that. He spoofed his authoritative image in a prominent role as King Richard in Mel Brooks' "Robin Hood: Men in Tights" (1993), reprised the role of Picard with his customary brio for the features "Star Trek: Generations" (1995) and "Star Trek: Insurrection" (1998), and offered a hilarious, scene-stealing turn as a very effeminate gay decorator in "Jeffrey" (1995), an enjoyable adaptation of the off-Broadway smash. He made an excellent Scrooge in the 1999 TNT remake of "A Christmas Carol", and a year later turned in a terrific performance as Professor Xavier, the wheelchair-bound leader of a group of superhero mutants, in the eagerly awaited big-screen version of the Marvel Comic "X-Men". Before that film's release, however, he grabbed headlines by making a curtain speech, slamming the producers for failing to promote his new Broadway play, "The Ride Down Mt. Morgan". The show's backers complained about the stunt to Actors Equity and Stewart was reprimanded and forced to apologize.

In addition to his work in theater and film, Stewart has kept busy in other realms of show business as well. His rich, cultured, instantly recognizable voice and his beautiful but unforced diction have kept him much in demand as a voice-over artist for recordings, TV commercials and documentaries. Among the latter he has hosted, narrated or at least spoken on episodes of the PBS series "Nova", another science series, "Space Age" (1992), the inaccurate but enjoyable film history "MGM: When the Lion Roars" (1992), and the CBS study of Native Americans, "500 Nations" (1995). He also lent his rich voice to the character of Napoleon the pig on the TNT animated adaptation of the George Orwell classic "Animal Farm" (1999). Acting on US TV occasionally, he enjoyed himself immensely in the title role of an adaptation of "The Canterville Ghost" (ABC, 1996), which he also co-produced, and as Captain Ahab in yet another retelling of "Moby Dick" (USA Network, 1998). He also won a Grammy for speaking the accompanying story for a 1996 recording of Prokofiev's "Peter and the Wolf". A playwright as well, Stewart has adapted several literary works for the radio and stage and has enjoyed particular success in Los Angeles and New York with his one-man version of "A Christmas Carol", in which he played more than 30 roles.