When war broke out in 1914, the 103rd Regiment contributed men to Calgary's Tenth Infantry Battalion. This battalion was formed in Valcartier, Quebec in September 1914 as part of the First Canadian Division, and went on to fight in every major Canadian engagement of the war.

The 10th Battalion sailed for the United Kingdom with the Canadian Expeditionary Force in late 1914, and after training on the rain drenched Salisbury Plain, they were shipped across the English Channel in early 1915 and into the trenches of France with the rest of the 1st Canadian Division.

The 1st Canadian Division saw their first major action at the Second Battle of Ypres in April 1915. This was the battle where chlorine gas was first used as a weapon of war on the Western Front. On the night of April 22-23rd, the 10th and 16th Battalions counter-attacked at Kitchener's Wood during the Battle of St. Julien.

More than 800 men of the 10th Battalion took part in the attack, and less than 200 survived. But because of their initiative, the German advance was halted. The attack by the 10th Battalion so impressed the Supreme Allied Commander at the time, Marshal Ferdinand Foch, he remarked that to him, it was the "finest act of the war".

As part of the 1st Canadian Division, the 10th Battalion saw action in several other campaigns, including Festubert and Mount Sorrel in 1916. They were also involved in the later stages of the Battle of the Somme at Thiepval Ridge and at Ancre Heights near the town of Albert, France.

In 1917, all four Canadian Divisions took part in the Battle for Vimy Ridge which was a major success for the Canadians. This was followed by the Battle for Hill 70, where the 10th Battalion was awarded their first Victoria Cross, 3 DSO's (Distinguished Service Order), 7 MC's (Military Cross), 9 DCM's (Distinguished Conduct Medals) and 60 MM's (Military Medals), giving the unit the distinction of receiving more medals than any other Canadian combat unit in a single action in the course of the First World War. In November 1917, the 10th Battalion took part in the Battle of Passchendaele.

During the last year of the First World War, the 10th Battalion saw action in the Battle of the Scarpe, the Battle of Amiens and the storming of the Hindenberg line. They crossed the Canal du Nord in late September 1918 and arrived in Mons during the last days of the war, and were there when the Armistice was declared. Over 1,300 soldiers of the 10th Battalion were killed during the First World War.

In 1921, the Calgary Regiment was organized and divided into two Battalions. The 1st Battalion became the Calgary Highlanders and the 2nd Battalion became The King's Own Calgary Regiment. In 1925, the Calgary Highlanders were allied with a distinguished Highland regiment in the British Army, the Argyll and Sutherland Highlanders.

The Calgary Highlanders were called upon again when war broke out with Germany in 1939. The regiment arrived in England in 1940 as part of the 2nd Canadian Division. They landed in Normandy a few weeks after D-Day, and saw action during the controversial and costly Battle of Verrières Ridge where, along with the Black Watch, the Calgary Highlanders took very heavy casualties.

The fighting moved north towards the Channel Ports of France and Belgium, and in September 1944 the 2nd Division forced a crossing of the Albert Canal, where the Calgary Highlanders played a major role.

The British Army captured Antwerp in September 1944, but the port could only be used once the final obstacle in the Scheldt Estuary, Walcheren Island, was cleared of German defenders. This task was given to the 2nd Canadian Infantry Division.

Walcheren Island was connected to the mainland by a narrow causeway, and The Calgary Highlanders, the Black Watch and the Régiment de Maisonneuve were called upon to capture it and establish a bridgehead on the island. After 3 days of bitter fighting, the island was finally taken with the help of a seaward landing by British Commandos. The Highlanders suffered over 60 casualties in the battle.

The Highlanders then fought in the Battle of Groningen, one of the largest divisional level urban battles fought by the Canadian Army. By the time the war ended, over 400 Highlanders had been killed, and over 1,600 men had been wounded.

Since the Second World War, the Calgary Highlanders have contributed soldiers to peacekeeping and NATO missions around the world, including Cyprus, Egypt, Golan Heights, Croatia, Bosnia, Cambodia, Kosovo, the former Republic of Yugoslavia, and Sudan. In 2008 and 2010, over 100 Calgary Highlanders have also served on NATO missions in Afghanistan.