Dorion was called to the bar in 1842 and was made Queen's Counsel in 1863. He entered politics in 1854 as member from Montreal of the legislature of the united province of Canada. He became leader of the Rouges, or young Liberals, of Canada East (formerly Lower Canada, now Quebec), working for reform. He gained a reputation for forthright expression of his opinions, especially in criticizing Conservative “deviousness.” His solution to the question of representation by population for Canada East and Canada West (now in Ontario) in the provincial legislature in 1856 was to recommend federation.

In August 1858 Dorion formed a dual administration with George Brown (Brown, George) of Canada West, but they resigned after three days. The alliance lost Dorion much support from the French Canadians, and he was defeated in Montreal but was later acclaimed for Hochelaga. In 1862 he helped form the John Sandfield Macdonald (Macdonald, John Sandfield)–Louis Victor Sicotte government, becoming provincial secretary; and in 1863–64 he was premier with John Sandfield Macdonald.

In the 1860s Dorion came to be a strong critic of Confederation, fearing for the liberties of the French Canadians; but he accepted the Dominion of Canada when it was created in 1867. Dorion served as dominion minister of justice (1873–74) in Alexander Mackenzie's Liberal government. He was appointed chief justice of Quebec in 1874, having acquired a great reputation in law. He was knighted in 1877.