LaFontaine was called to the bar in Lower Canada in 1828, and two years later he began his political career when elected to the provincial assembly for Terrebonne. He supported the French-Canadian grievances against the British governor in chief, but he did not condone the Rebellion of 1837. With a second outbreak of rebellion in 1838, LaFontaine was imprisoned but released without trial.

After the union of Upper and Lower Canada (1840) as Canada West and Canada East, LaFontaine took over the leadership of the French-Canadian Reformers. He declined the post of solicitor general offered by the first governor, Lord Sydenham, but responded to the request of the succeeding governor, Sir Charles Bagot, that LaFontaine form a ministry with Robert Baldwin, leader of the Reformers in Canada West (now Ontario). The ministry, formed in 1842, resigned within a year as a protest against the action of Bagot's successor, Sir Charles Metcalfe. After four years in opposition, LaFontaine formed a new administration with Baldwin under Lord Elgin, and they successfully established responsible government in Canada. LaFontaine's Rebellion Losses Bill (1849), which compensated those who suffered damages during the rebellion, precipitated riots in Montreal.

He retired from office in 1851 and was appointed chief justice of Canada East and president of the seigneurial court in 1853. He was made a baronet in 1854. (His two sons died in infancy, and the baronetcy became extinct upon his death.)