Mills made his film debut in The Midshipmaid (1932), and came to prominence as Colley in the 1939 film version of Goodbye, Mr Chips, opposite Robert Donat. He took the lead in Great Expectations in 1936, and subsequently made his career playing traditionally British heroes such as Sir Robert Falcon Scott in Scott of the Antarctic (1949). In 1941, he married the dramatist, Mary Hayley Bell, and their two daughters, Juliet and Hayley, are both successful actresses.
For his role as the village idiot in Ryan's Daughter (1970) - a complete departure from his usual style - Mills won an Academy Award for Best Supporting Actor. Altogether he appeared in over a hundred films. In 1976, he was knighted. In recent years, he has appeared on television only on special occasions.