On 6 August 1914, Details of the 71st York Regiment were placed on active service for local protective duty.[2]
On 10 August 1914, the 12th Battalion, CEF was authorized for service and on 30 September 1914, the battalion embarked for Great Britain. On 29 April 1915, the battalion was Redesignated as 12th Reserve Infantry Battalion, CEF and tasked to provide reinforcements for the Canadian Corps in the field. On 30 August 1920, the 12th Battalion, CEF was disbanded.[2]
On 22 December 1915, the 140th Battalion (St. John's Tigers), CEF was authorized for service and on 25 September 1916, the battalion embarked for Great Britain. On 2 November 1916, the battalion's personnel were absorbed by the depots of The Royal Canadian Regiment and the Princess Patricia's Canadian Light Infantry to provide reinforcements for the Canadian Corps in the field. On 27 July 1918, the 140th Battalion, CEF was disbanded.[2]