At the outbreak of World War I he joined the Canadian Army. His first stage appearance was in Siberia, where he entertained the American troops who were on occupation duty. Severely wounded in action in France he was shipped home where he eventually worked in the family business, selling farm implements. However, drawn to the theater, in 1922 he appeared on the London stage. His first movie role was High Treason in 1927, and he played Sherlock Holmes in The Speckled Band in the following year. Early in Massey's career, Abraham Lincoln's son, Robert Todd Lincoln (1843-1926), heard Massey perform and was struck by the close similarity of Massey's speaking voice to that of his father.
Despite being Canadian, Massey became famous for his quintessential American roles, as Abraham Lincoln in 1940's Abe Lincoln in Illinois (for which he was nominated for the Academy Award for Best Actor), in 1941's Santa Fe Trail, in which he played abolitionist John Brown, and as Lincoln again in 1962's How the West Was Won.
He rejoined the Canadian Army during World War II, and was wounded and invalided out in 1943. Following the war, he became an American citizen. He became well-known on television in the 1950s and 1960s, especially in his role as Doctor Gillespie in the series Doctor Kildare.
Massey has two stars on the Hollywood Walk of Fame, one for movies at 1719 Vine Street and one for television at 6708 Hollywood Blvd.