In the early stages of the First World War, Barrowclough volunteered for service in the New Zealand Expeditionary Force (NZEF), enlisting as a private in January 1915. He showed leadership potential and within four months had been commissioned as a second lieutenant. He departed for overseas service in October 1915 having been promoted to lieutenant and posted to the New Zealand Rifle Brigade. He saw brief service during the Senussi Campaign in the Middle East with the 2nd Battalion of the brigade.