Longmore set aside his life as a farm-hand in Geelong, Victoria, when he enlisted in the Australian Army with the branch of the Australian Imperial Force at age 21. Private (Pte) Roy Longmore, 3rd Reinforcements, 21st Battalion, of Bannockburn, Victoria enlisted on 13 July 1915; and he embarked from Melbourne aboard HMAT "Anchises" on 26 August 1915. After training in Egypt, was sent to the Battle of Gallipoli where he was a sapper with the 2nd Division, burrowing underneath the Turkish positions and laying mines. After evacuation from Turkey, he fought at Pozières, Armentières, and Villers-Bretonneux. He was wounded three times in France and was momentarily given up for dead when he was shot as the Armistice in November 1918, approached. He later recalled,