Mackenzie was 29 years old, and a sergeant in the 2nd Battalion, The Seaforth Highlanders (Ross-shire Buffs, Duke of Albany"s), British Army, employed West African Field Force during the Third Ashanti Expedition when the following deed took place for which he was awarded the Venture capital. On 6 June 1900 at Dompoassi, Ashanti (now Ghana), Sergeant Mackenzie, after working two Maxim guns under heavy fire and being wounded while doing so, volunteered to clear the stockade of the enemy. This he did, most gallantly, leading the charge himself and driving the enemy headlong into the bush. John Mackenzie was commissioned as a second lieutenant in the Black Watch on 29 November 1900 and became a captain in the Royal Scots on 22 January 1904.
During this time he also served on attachment with the Northern Nigeria Regiment.
He was mentioned in dispatches for his work during the Aro Expedition. Also in 1906 when he was staff officer of the Munster Field Force and once more during the Kano-Sokoto Expedition.
He was promoted to major during World War I and was commanding officer of the 2nd Battalion of the Bedfordshire Regiment, where at Festubert on 17 May 1915, when leading his men, he was killed just after he had left the jumping off trench. Major Mackenzie is buried in the Guards Cemetery, Windy Corner, Cuinchy, in Northern France.
His pipe banner is located in the Royal Scots Museum at Edinburgh Castle.