He originally pursued a military career, joining the Governor-General's Body Guard in 1912. He was transferred to France as part of the 4th Canadian Mounted Rifles in 1915, was wounded and decorated for valour (Military Cross) at the Battle of the Somme (1916), and was later wounded at Vimy Ridge.
After the war, Mackenzie left the army and returned to the family farm and took up farming. He entered municipal politics, serving as a member of the Woodbridge Village Council and later as reeve. He was elected member of the Legislature for York North under the Conservative banner in 1945 and represented the riding until his retirement at the 1967 provincial election.