After the outbreak of the First World War, Elliott joined the Australian Imperial Force (AIF), and formed and commanded the 7th Infantry Battalion, which he led in the landing at Anzac on 25 April 1915, and the Battle of Lone Pine in August. In March 1916, he became the commander of the newly formed 15th Infantry Brigade, which he led in the disastrous Battle of Fromelles in July 1916. In March 1917, the German retreat to the Hindenburg Line gave Elliott a rare chance to display his tactical acumen in an independent command as the 15th Brigade operated as an advance guard of the British Fifth Army. It fought in the Second Battle of Bullecourt in May 1917, and the Battle of Polygon Wood at the end of September 1917, when Elliott's leadership transformed a near-defeat into a victory. In the Second Battle of Villers-Bretonneux in April 1918, he won another famous victory.