[2] He established his professional reputation as a member of the Abbey Players in Dublin, while he achieved stage renown for his knowing interpretations of the works of Samuel Beckett.

He played O'Casey's brother Archie in Young Cassidy (1965), one of John Ford's later films, which the director had to abandon due to ill health.

[5][6] In 1954, he moved to London, where he became a member of the Royal Shakespeare Company, where he struck up a lasting friendship with actor Peter O'Toole, with whom he later appeared in Richard Brooks' Lord Jim (1965).

Other notable film appearances include the Ealing comedy The Titfield Thunderbolt (1953), Tony Richardson's Tom Jones (1963), David Lean's Doctor Zhivago (1965), Richard Lester's How I Won the War (1967), Peter Brook's King Lear, the leading role of Professor Collins in Wonderwall (1968), and Age of Consent (1969).

Shortly after completing work on The Exorcist, while in New York City appearing as Fluther in Seán O'Casey's The Plough and the Stars, MacGowran died at 54 from influenza after complications resulting from the London flu epidemic.