Trevelyan was born at the parsonage, Hindhead, Surrey, the younger son of the Reverend George Trevelyan, great-grandson of the Venerable George Trevelyan, Archdeacon of Taunton, third son of Sir John Trevelyan, 4th Baronet. His elder brother John Trevelyan was the Secretary of the Board of the British Board of Film Censors. The historian George Macaulay Trevelyan was a second cousin.