In between wars he was a member of the Army Sports Control Board 1935-1939 and took up service again in 1939. Harris was second-in-command of the 148th (Bedfordshire Yeomanry) Field Artillery Regiment sent to help defend Singapore in January 1942. He became a prisoner of war of the Japanese in Singapore and Siam for the next 3½ years. Harris was also promoted to Lieutenant-Colonel and commanding officer of the Bedfordshire Yeomanry in captivity when the Regiment's previous CO was killed trying to escape from Singapore.