A businessman and part-time Citizen Forces officer before the war, Leane was commissioned into the AIF and led a company of the 11th Infantry Battalion at the landing at Anzac Cove, Gallipoli, on 25 April 1915. He rose to temporarily command his battalion, and was made a Companion of the Distinguished Service Order (DSO), awarded the Military Cross, twice mentioned in despatches and wounded three times during the Gallipoli campaign. After returning to Egypt, the AIF was re-organised, and Leane was appointed as the commanding officer of the newly formed 48th Battalion, which soon after was transported to the Western Front in France and Belgium. Important battles that the 48th were involved in under his command included the Battle of Pozières in 1916, and the First Battle of Bullecourt and First Battle of Passchendaele in 1917. During the latter battle, he was severely wounded. During 1916–1917, he was mentioned in despatches three more times, was made a Companion of the Order of St Michael and St George and awarded a bar to his DSO.