Priestley served in the army during the First World War, volunteering to join the 10th Battalion, the Duke of Wellington's Regiment on 7 September 1914, and being posted to France as a Lance-Corporal on 26 August 1915. He was badly wounded in June 1916, when he was buried alive by a trench-mortar. He spent many months in military hospitals and convalescent establishments, and on 26 January 1918 was commissioned as an officer in the Devonshire Regiment, and posted back to France late summer 1918. As he describes in his literary reminiscences, Margin Released, he suffered from the effects of poison gas, and then supervised German prisoners of war, before being demobilized in early 1919.