Mackenzie was born in Logierait, Perthshire, Scotland. He left school at the age of 13, following his father's death to help his widowed mother, and trained as a stonemason. Mackenzie immigrated to Canada when he was 19, settling in what became Ontario. His masonry business prospered, allowing him to pursue other interests – such as the editorship of a pro-Reformist of a newspaper called the" Lambton Shield". Mackenzie was elected to the Legislative Assembly of the Province of Canada in 1861, as a supporter of George Brown.