Harris emigrated to Southern Rhodesia in 1910, aged 17, but returned to England in 1915 to fight in the European theatre of the First World War. He joined the Royal Flying Corps, with which he remained until the formation of the Royal Air Force in 1918, and he remained in the Air Force through the 1920s and 1930s, serving in India, Mesopotamia, Persia, Egypt, Palestine, and elsewhere. At the outbreak of the Second World War in 1939, Harris took command of No. 5 Group RAF in England, and in February 1942 was appointed head of Bomber Command.