Grant joined the Australian Imperial Force on 16 March 1915, taking command of the 11th Light Horse Regiment. Part of the 4th Light Horse Brigade, it was sent to Egypt dismounted and there broken up on 26 August 1915. The 11th Light Horse Regiment was sent to Gallipoli, and there itself broken up, with a squadron being attached to each of the 2nd, 5th and 9th Light Horse Regiments. When the commander of the 9th Light Horse, Lieutenant Colonel Reynell was killed in the fighting for Hill 60 on 29 August 1915, Grant took over command of his regiment. He remained with the 9th Light Horse until he was sent for duty at the rest camp on 2 December 1915.