He originally pursued a military career, joining the Governor-General"s Body Guard in 1912. After the war, Mackenzie left the army and returned to the family farm and took up farming. The Woodbridge branch of the Royal Canadian Legion is named in his honour as is Major Mackenzie Drive in the Regional Municipality of York, as well as Alexander Mackenzie High School.
Mackenzie died in 1970.
He entered municipal politics, serving as a member of the Woodbridge (Ontario) Village Council and later as reeve. He was elected member of the Legislature for York North under the Progressive Conservative banner in 1945 and represented the riding until his retirement at the 1967 provincial election. Mackenzie also served as a member of the Metropolitan Toronto and Region Conservation Authority (1961-1970), and was a charter member of the Woodbridge Horticultural Society.
