[1] He was educated at Stonyhurst College, a Jesuit public school in Lancashire in the North of England, where he won prizes for elocution and played rugby and cricket.

His breakthrough as a stage actor came when he was cast as Mat Burke in Anna Christie with Flora Robson and Alexander Knox.

On the West End, he appeared in Volpone with Donald Wolfit, at the Westminster Theatre, and played Malcolm in Michel Saint-Denis's production of Macbeth at the Old Vic, with Laurence Olivier in the title role.

He appeared regularly in Old Vic productions, especially Shakespearean plays, and also performed with the Longford players during their 1937–8 London season, MacGinnis made his 1935 film debut in Turn of the Tide.

In 1944, he played Captain MacMorris opposite Olivier in Henry V. However, that same year he put his acting career on hiatus to join the British Royal Navy as a surgeon.

One critic called MacGinnis' performance "magnificent... given reverential, straightforward, honest, sincere treatment, as well as eschewing anything savoring of sensationalism.

At the time of his death, he was considering coming out of retirement to appear to narrate his old friend Michael Powell's film Return to the Edge of the World (1978).