On 5 August 1914, Spencer was commissioned as a second lieutenant in the 1st Regiment of Life Guards, was promoted to lieutenant on 21 October 1914, appointed an aide-de-camp on 9 May 1917, and promoted to captain on 15 June 1917. When 1st Life Guards merged with the 2nd Life Guards on 18 November 1922, Spencer was appointed a captain in the new regiment. He retired from the army on 20 September 1924, but remained a member of the Regular Army Reserve of Officers until reaching the mandatory retirement age on 2 June 1943.