Keysor had only been in Australia for about three months when the First World War broke out. He enlisted in the Australian Imperial Force on 18 August 1914 as a private and was assigned to the 1st Battalion, which was forming at Randwick. On 18 October 1914 he embarked for overseas service among the first contingent. Initially he served in Egypt where the Australians were involved in the defence of the Suez Canal against the Turks, but on 25 April 1915, he landed at Gallipoli where he took part in the subsequent fighting on the peninsula. On 20 June 1915 he was promoted to lance corporal, before taking part in the Battle of Lone Pine in August.