In 1915, Croak enlisted in the Canadian Army and volunteered for service abroad with the Canadian Expeditionary Force. Posted to the 55th Battalion as a private, he embarked for Europe in November 1915. He soon transferred to the 13th Battalion, which was serving on the Western Front as part of 3rd Brigade, 1st Canadian Division. Through 1917 and the early part of 1918, Croak participated in several engagements as part of 13th Battalion; these included the Battles of Vimy Ridge, Hill 70 and Passchendaele.