On October 19th, 1914, authority was granted to organize an infantry battalion to be part of the 2nd Canadian Contingent. Lt. Col. W. St. Pierre Hughes (later Brigadier General in Command of the 10th Canadian Infantry Brigade) mobilized and commanded the 21st. Battalion CEF, which The Princess of Wales' Own Regiment perpetuates. Other formations to which The Princess of Wales' Own Rifles contributed officers and men included the 59th Bn., the 146th Bn., and the 253rd Bn., all of the CEF.
Eleven Battle Honours were added to the Colours during the three years engaged in World War I. The Commanding Officer of the 21st. Bn. at the end of the War was Lt. Col. Henry Edward Pense, DSO,MC,MID, an officer of The Princess of Wales' Own Rifles.
The 21st. Bn. (Eastern Ontario) was the fourth Bn. in the 4th Infantry Brigade of the 2nd Canadian Division of the Canadian Corps. the other three Bns. were the 18th (Western Ontario), the 19th (Central Ontario) and the 20th (Central Ontario).