He was born in Sainte-Geneviève-de-Berthier, Lower Canada, the son of Louis Mousseau, the son of Alexis Mousseau, and Sophie Duteau, dit Grandpré. Mousseau was first elected to the Canadian House of Commons as a Conservative Member of Parliament in the 1874 election for the riding of Bagot, and was re-elected three times. In 1880, he was elevated to the Cabinet of Prime Minister Sir John A. Macdonald, serving first as president of the Queen's Privy Council of Canada, and then as Secretary of State for Canada.
Mousseau left federal politics to become the sixth Premier of the province of Quebec from July 31, 1882, until his resignation on January 22, 1884, after being appointed as a judge. He died in Montreal in 1886.
His brother Joseph Octave Mousseau was also a member of the Canadian House of Commons.