In October 1914, Antill volunteered for overseas service with the Australian Imperial Force. As a regular officer, he was assigned to the 3rd Light Horse Brigade as brigade major, under the command of Colonel Frederic Hughes. He departed for the Middle East on 25 February 1915 on the Transport A16 "Star of Victoria", reaching Egypt where the brigade undertook further training. In May, the light horsemen were sent to Anzac Cove as dismounted reinforcements; initially, Antill's brigade undertook mainly defensive duties. However, in an effort to break the deadlock they were committed to the August Offensive. During the fierce fighting at The Nek, Antill, now in temporary command of the brigade, refused a request from the commander of the 10th Light Horse, Lieutenant Colonel Noel Brazier, to cancel the third wave. Hughes had gone forward to observe the attack, leaving Antill in charge. Due to a communications breakdown, Antill believed Hughes had gone forward to lead the brigade forward and therefore ordered the attack to continue. Part of the fourth wave also went over the top, before Brazier and some officers from the 8th Light Horse Regiment reached Hughes in time to call off any further waves. Out of around 500 men committed to the attack, more than half became casualties, with 234 being killed and 138 wounded. Most of those that were killed, died within only a few metres of the Australian trench line having come under heavy, unsuppressed fire.