McLaglen was born in England. His father, a bishop, moved the family to South Africa when McLaglen was a child. He left home at fourteen to join the army and fought in the Boer War. Four years later, he moved to Canada, where he earned a living as a wrestler and heavyweight boxer, with several notable wins in the ring. One of his most famous fights was against Heavyweight Champion Jack Johnson, who defeated him in six rounds. Between bouts, he toured with a circus, which offered $25 to anyone who could go three rounds with him. He returned to England in 1913 and served with the Royal Irish Fusiliers during World War I. He also continued boxing, and was named Heavyweight Champ of the British Army in 1918. After the war, he began taking roles in British silent films.
McLaglen's career took a turn in the 1920s, when he moved to Hollywood. He became a popular character actor, with a particular knack for playing drunks. The highlight of his career was an Academy Award for Best Actor for his role in The Informer (1935), based on a novel by Liam O'Flaherty. He was nominated again, this time for Best Supporting Actor, for his role opposite John Wayne in The Quiet Man (1952). He was especially popular with director John Ford, who frequently included McLaglen in his films. Toward the end of his career, McLaglen made several guest appearances on television, particularly in Western series such as Have Gun Will Travel and Rawhide.
He died in 1959. He had by that stage become a naturalized U.S. citizen.