While working as a clerk at the British Steamship Company in the City of London, he joined the London Scottish Regiment in 1909 as a Territorial Army soldier. At the outbreak of World War I, he was mobilised, and sent to France in September 1914. On 31 October 1914, at the Battle of Messines, Colman was seriously wounded by shrapnel in his ankle, which gave him a limp that he would attempt to hide throughout the rest of his acting career. As a consequence, he was mustered out as invalid in 1915. His fellow Hollywood actors Claude Rains, Herbert Marshall, Cedric Hardwicke, and Basil Rathbone all saw service with the London Scottish in the war.