William Lyon Mackenzie, Scottish-Canadian journalist and politician, 1st Mayor of Toronto (d. 1861)
William Lyon Mackenzie (March 12, 1795 – August 28, 1861) was a Scottish-born Canadian-American journalist and politician. He founded newspapers critical of the Family Compact, a term used to identify elite members of Upper Canada. He represented York County in the Legislative Assembly of Upper Canada and aligned with Reformers. He led the rebels in the Upper Canada Rebellion; after its defeat, he rallied American support for an unsuccessful invasion of Upper Canada as part of the Patriot War. Although popular for criticising government officials, he failed to implement most of his policy objectives. He is one of the most recognizable Reformers of the early 19th century.

Raised in Dundee, Scotland, Mackenzie emigrated to York, Upper Canada, in 1820. He published his first newspaper, the Colonial Advocate in 1824, and was elected a York County representative to the Legislative Assembly in 1827. York became the city of Toronto in 1834 and Mackenzie was elected its first mayor; he declined the Reformers' nomination to run in the 1835 municipal election. He lost his re-election for the Legislative Assembly in 1836; this convinced him that reforms to the Upper Canadian political system could only happen if citizens initiated an armed conflict. In 1837, he rallied farmers in the area surrounding Toronto and convinced Reform leaders to support the Upper Canada Rebellion. Rebel leaders chose Mackenzie to be their military commander, but were defeated by government troops at the Battle of Montgomery's Tavern.

Mackenzie fled to the United States and rallied American support to invade Upper Canada and overthrow the province's government. This violated the Neutrality Act, which prohibits invading a foreign country (with which the United States is not at war) from American territory. Mackenzie was arrested and sentenced to eighteen months imprisonment. He was jailed for more than ten months before he was pardoned by the American president Martin Van Buren. After his release, Mackenzie lived in several cities in New York State and tried to publish newspapers, but these ventures failed. He discovered documents that outlined corrupt financial transactions and government appointments by New York State government officials. He published these documents in two books. The parliament of the newly created Province of Canada, formed from the merger of Upper and Lower Canada, granted Mackenzie amnesty in 1849 and he returned to Canada. He represented the constituency of Haldimand County in the province's legislature from 1851 to 1858. His health deteriorated in 1861 and he died on August 28.