Born Patrick McGee in Armagh, Northern Ireland, he changed his name to Magee for the stage, after attended attending St. Patrick's Roman Catholic College. His first stage experience in Ireland was with Anew McMaster's touring company, performing the works of Shakespeare. It was here that he first worked with Pinter.
He was then brought to London by Tyrone Guthrie for a series of Irish plays. In 1957 he met Beckett and recorded some of his prose for BBC radio. Beckett was so excited with his voice that he wrote Krapp's Last Tape especially for him (it was filmed by the BBC in 1972). Beckett's biographer Anthony Cronin wrote that "there was a sense in which, as an actor, he had been waiting for Beckett as Beckett had been waiting for him."
In 1964 he joined the Royal Shakespeare Company, where Pinter, directing his own play The Birthday Party, specifically requested him for the role of McCann, and stated he was the strongest in the cast. In 1965 he appeared in Marat/Sade, and when the play transferred to Broadway it won him a Tony Award. He also appeared in the 1966 RSC production of Staircase opposite Paul Scofield.
Early film roles included Joseph Losey's The Criminal (1960) and The Servant (1963), the latter written by Pinter. He also appeared in Zulu (1964), Séance on a Wet Afternoon (1964), and the film versions of Marat/Sade (1967) and The Birthday Party (1968). But he is perhaps best known for his role as the victimised writer Mr. Alexander in Stanley Kubrick's film A Clockwork Orange.
He went on to appear in Kubrick's Barry Lyndon (1975), but was most often seen in horror films. These included Roger Corman's The Masque of Red Death (1964), and Boris Karloff vehicle Die, Monster, Die! (1965) for AIP; The Skull (1965), Tales from the Crypt (1972) and Asylum (1972) for Amicus Productions; and Demons of the Mind for Hammer Film Productions.
He was married to Belle, a girl from his home town Armagh, and they had twins Mark and Caroline, who were born in London in February 1961. Always known as a heavy drinker, Magee died of a heart attack in 1982 at age 60.