He left Australia on the transport ship "Medic" in November 1914 as a private in the 1st Divisional Ammunition Column, but transferred to the 16th Battalion in October 1915, where he served in the Gallipoli Campaign. Cumming transferred to the 48th Battalion in March 1916, as a second lieutenant, and was sent to serve on the Western Front with his unit. He was wounded in action at the Battle of Pozières, which necessitated a period of recovery in England, but was promoted to the rank of lieutenant before his return to France.