The son of a carriage maker, Garneau left school at the age of 14 and entered the court clerk's office and two years later a notary's firm, becoming a notary himself in 1830. He was in London from 1831 to 1833 as secretary to D.B. Viger, a Canadian political delegate. Back in Quebec he wrote poetry, started two short-lived periodicals, dabbled in history, and in 1837 became a bank cashier. Active in local politics, Garneau was appointed translator to the legislative assembly (1843) and was the city clerk of Quebec (1844–64).

Garneau's Histoire du Canada (1845–48), predominantly a political and military account of early Quebec, includes tales of pioneering men and women and descriptions of the major civil, political, and religious leaders. An attempt to conserve Quebec's religion, language, and laws, the work met with great success and inspired a reawakening of interest by poets, novelists, and other historians in French-Canadian culture.