Mackenzie arrived in UPPER CANADA in 1820 and, after a few years in business at Dundas, moved to QUEENSTON. In May 1824 he published the first issue of the Colonial Advocate, which immediately became a leading voice of the new Reform movement. To be closer to the provincial Parliament, Mackenzie moved his operation to York [Toronto] in the fall of 1824. His forthright and forceful manner together with his ardent denunciation of the FAMILY COMPACT contributed much to his popularity, and in 1828 he was easily elected to the House of Assembly for York County.

In 1832 he visited England to present his political supporters' grievances before the imperial government. The sympathetic hearing he received outraged Upper Canadian conservatives. Moreover, Mackenzie's venomous attacks on the local oligarchy brought reprisals in the form of libel suits, threats and physical assaults, as well as an attack on his printing office, which left his press wrecked and the type thrown into the lake. The diminutive Scot's scathing attacks on his opponents also led to his repeated expulsion from the Assembly, although he was continually re-elected by his rural constituents. In 1834, when the Reformers won a majority on the newly created Toronto City Council, he was elected its first mayor. At the end of 1834, he was elected to the provincial Parliament again. However, he was defeated at the polls in 1836, and in December 1837 an embittered Mackenzie turned his mind to armed revolt.

On December 6, convinced that he would gain spontaneous support, he led an erratic expedition down Yonge Street towards Toronto, seemingly more intent on damaging the property of Tory supporters than taking control of the government. As the force neared Toronto it was dispersed by a few shots from loyalist guards. On December 7 loyalists marched north to Montgomery's Tavern and easily defeated the rebels. Mackenzie fled to the US and tried to muster a new scheme from NAVY ISLAND in the Niagara River. Canadian militia bombarded the island and sank the rebel supply ship Caroline.

Mackenzie moved to New York where he founded Mackenzie's Gazette. However, he was convicted of violation of the US neutrality laws and imprisoned for a year, falling ill and deeper in debt. He spent the next 10 years in the US, eventually finding employment as a correspondent for the New York Daily Tribune. During exile he wrote several books, including The Sons of the Emerald Isle (1844), The Lives and Opinions of Benjamin Franklin Butler and Jesse Hoyt (1845) and The Life and Times of Martin Van Buren (1846).

Mackenzie returned to Canada in 1849 following a government pardon. Undaunted, he quickly resumed both his journalistic and his political careers, serving with characteristic energy as MLA for Haldimand until retirement in 1857 and occasionally publishing a political squib usually entitled Mackenzie's Weekly Message. The fiery and principled Scot died at his home on Bond Street, now one of Toronto's historic sites and museums.