Born in Scotland, Mackenzie came to Upper Canada in 1820. He became a prominent radical journalist and was first elected to the assembly in 1828, building up a strong popular following. He was the first mayor of the city of Toronto in 1834. Frustrated by political setbacks, Mackenzie led an abortive rebellion in 1837, and fled to the United States. From there he watched the achievement of Canadian self-government, which he had sought ardently but without success. Returning under amnesty in 1850, he sat in Parliament again until 1858.