While still studying at Oxford, Mackenzie was commissioned as a second lieutenant in the Auxiliary Forces in May 1908. After graduating from Oxford Mackenzie chose a career in the British Army. Having been attached to the East Lancashire Regiment as a member of the Auxiliary Forces, he was assigned to the King's Royal Rifle Corps in July 1911, . where he was attached to the 4th Battalion and served in British India. He was promoted to lieutenant in March 1914 and was on home leave when the First World War was declared in at the end of July 1914. He was seconded to the Rifle Brigade at the commencement of hostilities, where he was sent to France and took part in the Battle of the Aisne in September 1914. Involved in a dawn assault on German trenches near Soupir on 25 September, Mackenzie was wounded in the assault, but was seen to return to his feet to lead his platoon and was a short while later wounded for a second time, this time fatally. His body was recovered from the battlefield and buried at the Montcornet Cemetery.