Mackenzie was born on 28 January 1822 in Logierait, Perthshire, Scotland, the son of Mary Stewart (Fleming) and Alexander Mackenzie, Sr (born 1784), who were married in 1817. The site of his birthplace is known as Clais-'n-deoir "The Hollow of the Weeping", where families said their goodbyes as the convicted were led to nearby Gallows Hill. The house in which he was born was built by his father and is still standing in 2019. He was the third of 10 boys, seven of whom survived infancy. Alexander Mackenzie, Sr., was a carpenter and ship's joiner who had to move around frequently for work after the end of the Napoleonic Wars in 1815. Mackenzie's father died on 7 March 1836 and at the age of 13, Alexander Mackenzie, Jr., was thus forced to end his formal education to help support his family. He apprenticed as a stonemason and met his future wife, Helen Neil, in Irvine, where her father was also a stonemason. The Neils were Baptist and shortly thereafter, Mackenzie converted from Presbyterianism to Baptist beliefs. Together with the Neils, he immigrated to Canada in 1842 to seek a better life. Mackenzie's faith was to link him to the increasingly influential temperance cause, particularly strong in Canada West where he lived, a constituency of which he was to represent in the Parliament of Canada.