After some professional training he joined the Royal Shakespeare Company in 1966 and then 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 Sejanus in I, Claudius and Karla in Tinker, Tailor, Soldier, Spy, and took the romantic male lead in the BBC adaptation of Mrs Gaskell's North and South (wearing a hairpiece).
In 1987 Stewart went to Los Angeles to star as Captain Jean-Luc Picard in Star Trek: The Next Generation (1987-1994). From 1994 he also portrayed Picard in the movie spin-offs Star Trek: Generations, Star Trek: First Contact, Star Trek: Insurrection, and Star Trek: Nemesis.
Although that film series has concluded, he has also starred in X-Men and X2 as Professor Charles Xavier and the film's success has appeared to have resulted in another lucrative regular role in the burgeoning superhero film series.