The battalion was authorized on 7 November 1914 and embarked for Britain on 6 May 1915.

[2] The 21st Battalion had five Officers Commanding: The Ottawa Branch of the 21st Battalion Association erected a memorial plaque at St. Andrew's Presbyterian Church (Ottawa) which is dedicated to Brigadier General William St Pierre Hughes, DSO, VD, first Commanding Officer of the battalion.

[3] The 21st Battalion (Eastern Ontario), CEF, is perpetuated by The Princess of Wales' Own Regiment.

[4] The 21st Battalion was awarded the following battle honours: Capt.

Herbert W. McBride, who served with the 21st Battalion, wrote two books about his experiences as a member of the unit during the Great War as a sniper and machine gunner: "A Rifleman Went To War" and "The Emma Gees.