He was educated at Stonyhurst College in England, and studied medicine at Dublin University.
He qualified as a house surgeon. During the Second World War, MacGinnis served as surgeon in the Royal Navy. MacGinnis played a German sailor in the British war film 49th Parallel (1941) with Laurence Olivier, Leslie Howard, Eric Portman and Raymond Massey.
He portrayed Captain MacMorris in Olivier"s version of Henry V (1944) and the title character in the film Martin Luther (1953).
He played the villainous Julian Karswell opposite American actor Dana Andrews in the British horror film Night of the Demon (1957, released in the United States as Curse of the Demon). He also played Zeus opposite Honor Blackman"s Hera in Jason and the Argonauts (1963), played the arch-criminal A. J. Kent in an episode of Danger Manitoba entitled "Battle of The Cameras" (1965).
And had a supporting role in John Huston"s film The Kremlin Letter (1969). MacGinnis returned to medical practice during the 1970s.
He died of cancer in Haverfordwest, Wales in 1977, aged 63.