Mackenzie married Helen Neil (1826-1852) in 1845 and with her had three children, with only one girl surviving infancy. In 1853, he married Jane Sym (1825-1893).
In Canada, Mackenzie continued his career as a stone mason, building many structures that still stand today. He began working as a general contractor and it is in doing this that he earned his reputation for being a hard working, honest man as well as having a working man's view on fiscal policy.
Mackenzie involved himself in politics almost from the moment he arrived in Canada. He campaigned relentlessly for George Brown, owner of the Reformist paper The Globe, in the 1851 election, helping him to win a seat in the assembly. In 1852 Mackenzie became editor of a different reformist paper, the Lampton Shield. As editor, Mackenzie was perhaps a little too vocal, leading the paper to a suit of law for libel against the local conservative candidate. The paper lost the suit and was forced to fold due to financial hardship. He was elected to the Legislative Assembly himself as a supporter of George Brown in 1861.
When the Macdonald government fell due to the Pacific scandal in 1873, the Governor General, Lord Dufferin, had to call on someone to form a government. Mackenzie had been chosen as the leader of the Liberal Party just a few months before and so was called upon to form a new government. Mackenzie formed a government and then asked the Governor General to call an election for January 1874. The Liberals won, and Mackenzie remained prime minister until the 1878 election when Macdonald's Conservatives returned to power with a majority government.
It was unusual for a man of Mackenzie's humble origins to attain such a position in an age which generally offered such opportunity only to the privileged. Lord Dufferin, the current Governor General, expressed early misgivings about a stonemason taking over government. But on meeting Mackenzie, Dufferin revised his opinions: "However narrow and inexperienced Mackenzie may be, I imagine he is a thoroughly upright, well-principled, and well-meaning man."
Mackenzie also served as Minister of Public Works and oversaw the completion of the Parliament Buildings. While drawing up the plans, he included a circular staircase leading directly from his office to the outside of the building. This clever addition allowed him to escape the patronage-seekers waiting for him in his ante-chamber. Proving Dufferin's reflections on his character to be true, Mackenzie disliked intensely the patronage inherent in politics. Nevertheless, he found it a necessary evil in order to maintain party unity and ensure the loyalty of his fellow Liberals.
In keeping with his democratic ideals, Mackenzie refused the offer of a knighthood three times. His pride in his working-class origins never left him. Once, while touring Fort Henry as prime minister, he asked the soldier accompanying him if he knew the thickness of the wall beside them. The embarrassed escort confessed that he didn't and Mackenzie replied, "I do. It is five feet, ten inches. I know, because I built it myself!"
As Prime Minister, Alexander Mackenzie strove to reform and simplify the machinery of government. He introduced the secret ballot; created the Supreme Court of Canada; established the Royal Military College of Canada in Kingston in 1874; created the Office of the Auditor General in 1878; and struggled to continue progress on the national railway. After his government's defeat, Mackenzie remained Leader of the Opposition until 1880, when he relinquished the party leadership to Edward Blake. However, he remained as a Member of Parliament until his death in 1892 from a stroke that resulted from hitting his head during a fall. He died in Toronto and is buried in the Lakeview Cemetery, Sarnia.
At the time, it was customary for the monarch to knight all Canadian Prime Ministers but Mackenzie declined all offers of a knighthood. He was the only Canadian Prime Minister not to be knighted until Arthur Meighen took office in 1920.