Lieutenant MacKenzie joined the Cumberland Highlanders as a private in 1936 and went overseas with the Royal Canadian Engineers in June 1941. He served with this unit in England until February 1944, when he was recommended for a commission. He was sent to Sandhurst, one of the first Canadian officers to be educated at the English school under British officers. He graduated on September 21, 1944. In October 1944 he was conscripted to Belgium where he was attached to the Support Company of the Royal Winnipeg Rifles.

He served with this regiment during the heavy battles surrounding the Scheldt and Rhineland Campaign. During the heavy fighting for the Schipbeek Canal on April 7, 1945, MacKenzie and his pioneer section were responsible for the crossing. Under heavy German machine gun fire, MacKenzie manages to lay four planks across the canal so that the rest of his company can cross. During this action, MacKenzie was wounded by fragments of German hand grenades. For his brave efforts at the Schipbeek Canal, he was nominated for a Military Cross, which he would unfortunately receive posthumously.

On Sunday morning, April 22, 1945, the regiment was ordered to attack Appingedam in the north of the Netherlands. There is heavy fighting on the outskirts of the city, with the leading companies coming under heavy fire from the German artillery positions along the coast of the port of Delfzijl.

Lieutenant MacKenzie was sent forward with his Support Company to clear mines on the road leading from Tjamsweer (near Appingedam) to the city. Two of his men were wounded when they were caught in machine gun fire as the unit tried to cross a bridge. Lieutenant. MacKenzie made his way to the bridge under murderous German machine gun fire to evacuate the first wounded man. A little later he returned for the second wounded man and managed to reach a place of safety, while many of his comrades looked on in amazement at his daring action.

Shortly afterwards he was killed by a sniper's bullet fired from a nearby building. He was temporarily buried in Loppersum. Thanks to MacKenzie's actions, the city of Appingedam was liberated on April 23, 1945. He is said to be the only officer from Springhill, Nova Scotia to be awarded a Military Cross during World War II.