He opposed Confederation (1867) of the British North American provinces and founded (1871) the Parti National to represent the French element in Canada. He served in the Canadian House of Commons from 1872 to 1874. Elected to the legislative assembly of the province of Quebec in 1879, he became leader of the Liberal Party of Quebec in 1883, and in 1886, in the wake of the Riel Rebellion, defeated the Conservatives to become Premier of Quebec. His immense popularity was eventually undermined by charges of corruption in connection with railroad subsidies, and in 1891 he was dismissed from office and defeated in the subsequent election. The new government acquitted Mercier of the charges.