Barrett-Lennard's father, Sir Fiennes Cecil Arthur Barrett-Lennard (1880-1963), was a British soldier, who fought in the Boer War and in East Africa in the First World War, and became a judge in Malaya, then Johore and Kedah, and finally Chief Justice of Jamaica. He was educated at Radley College in Oxfordshire. He and his mother converted to Roman Catholicism in the 1930s. He became a teacher at St Philip's prep school in Kensington, and was due to join the London Oratory when the Second World War broke out.