After his national service as a gunner in the Airborne Division, Williamson made his professional debut with the Dundee Repertory Theatre in 1960 and the following year appeared with the Arts Theatre in Cambridge. 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. 1964 also saw him appearing as Vladimir in Samuel Beckett's "Waiting for Godot" at the Royal Court. He starred in the film version in 1968. 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. Faithfull later stated in her autobiography "Faithfull" that she and Williamson had had an affair while filming "Hamlet".