On 1 September 1939, the Calgary Highlanders were ordered to mobilize for the Second World War. The regiment trained in Calgary until the summer of 1940 when it departed for CFB Shilo, Manitoba. The Calgary Highlanders, CASF (Canadian Active Service Force) joined the Second Canadian Division there, and the 2nd Battalion was raised in Calgary for part-time service.
In September 1940, the 1st Battalion arrived in England.
During the Dieppe Raid of August 1942, the mortar platoon commanded by Lieutenant FJ Reynolds was attached to the 5th Canadian Infantry Brigade but stayed offshore during the raid. Sergeants Lyster and Pittaway were decorated with a Mention in Despatches for their part in shooting down two German aircraft during the raid, and one officer of the regiment was killed while ashore with a brigade headquarters.
The Calgary Highlanders pioneered battle drill for the Canadian Army, which was a realistic system of training infantry for the hardships of modern war. They themselves learned battle drill from the British 47th Division.
On 6 July 1944, one month after the Normandy landings, the regiment landed in France. In Operation Spring, the Calgary Highlanders were part of the Battle of Verrières Ridge, along with the Black Watch, in which the regiment took heavy casualties. The unit saw extensive action in Normandy, marched through Dieppe with the 2nd Division in September 1944 as liberators, then moved on to the fighting for the Channel Ports. By the end of September the regiment was in Belgium and forced a crossing of the Albert Canal, northeast of Antwerp.
The regiment saw extensive fighting in the Netherlands in October 1944, opening the way to South Beveland, and then west to the Walcheren Island Causeway where the brigade fought an extended battle beginning on Hallowe’en night.
From November to February 1945 the regiment wintered in the Nijmegen Salient, then was back in action in the Rhineland fighting, clearing the last approaches to the River Rhineitself. Fighting resumed on the far bank in March, and city fighting in Doetinchem and Groningen followed. The regiment ended the war on VE Day on German soil.
The Victory Campaign had cost The Calgary Highlanders over 400 men killed, from a war establishment of just over 800 men. Several times that many were wounded in action.