In 1962 he made his London debut as Flute in Tony Richardson's production of A Midsummer Night's Dream at the Royal Court Theatre.

His first major success came in 1964 with John Osborne's Inadmissible Evidence, for which he was nominated for a Tony Award when it transferred to Broadway the following year.

Williamson's Hamlet for Tony Richardson at the Roundhouse caused a sensation; it was later transferred to New York and made into a film, with a cast including Anthony Hopkins and Marianne Faithfull.

Director John Boorman cast him opposite Helen Mirren as Morgana over the protests of both actors; the two had previously appeared together on stage in Macbeth, with disastrous results, and they disliked each other intensely.

Additionally, he portrayed an MI6 bureaucrat in The Human Factor (1979) (adapted from a novel by Graham Greene); an alcoholic attorney in I'm Dancing as Fast as I Can (1982); a colonel in the Cincinnati Gestapo in the Neil Simon comedy The Cheap Detective; Lord Louis Mountbatten in Lord Mountbatten - The Last Viceroy (1985); the dual roles of Dr. Worley/The Nome King in Return to Oz (1985); Father Morning in The Exorcist III (1990); Badger in the 1996 movie adaptation of Kenneth Grahame's Wind in the Willows; and as Cogliostro in the 1997 movie adaptation of Todd McFarlane's comic book Spawn.

During the Philadelphia tryout of Inadmissible Evidence, a play in which he delivered a performance that would win him a Tony Award nomination in 1965,[13] he punched the equally mercurial producer David Merrick.

[14] In the early 1970s, Williamson left The Dick Cavett Show prior to a scheduled appearance, leaving the host and guest Nora Ephron to fill the remaining time.

[13] Following a late-night chat show appearance in which he showcased his singing talents, Williamson released an album of songs in 1971 on the CBS label (S 64045).

[18] According to his official website, Williamson re-edited the original script himself, removing many occurrences of "he said", "she said", and so on, as he felt that an over-reliance on descriptive narrative would not give the desired effect; he performed each character in a distinctive voice.