The battalion was authorized on 7 November 1914 and embarked for Britain on 6 May 1915.
It disembarked in France on 15 September 1915, where it fought as part of the 4th Canadian Brigade, 2nd Canadian Division in France and Flanders.
The battalion was disbanded on 30 August 1920.
The 21st Battalion recruited in Eastern Ontario and was mobilized at Kingston, Ontario.
The 21st Battalion was awarded the following battle honours:
Mont Sorrel
Somme 1916, 1918
Flers-Courcelette
Thiepval
Ancre Heights
Arras 1917, 1918
Vimy Ridge 1917
Hill 70
Passchendaele
Amiens
Scarpe 1918
Drocourt-Quéant
Hindenburg Line
Canal du Nord
Cambrai 1918
Pursuit to Mons
France & Flanders 1915, 1918
Numeral Collar Badge
Formation Patche
Cap Badge
Bailey, Peter # 455717
Baker, Albert Victor # 633575
Campbell, William MM # 633222
Carey, David Philip # 445213
Cheal, Frederick James # 59156
Crooks, Samuel Thomas MC # 469394
Darling, George Henry # 636487
DesRoches, Eddie Hubert MM # 444453
Grant, Thomas Ewart # 724269
Lackie, Orris Dorin # 675929
Millard, Roy # 123540
Norman, Heny Foley aka Baxter #69100
Pearson, Thomas # 144959
Salisbury, Roy Dufferin # 123495
Shuttleworth, Henry Charles # 409488
Smeed, Edward George # 210878
Smith, William Cadwell # 454873
Spinks, Frederick Joseph # 675906
Vance, James # 675282