After articling in the chambers of Louis Conrad Pelletier, he was called to the Quebec bar on 9 July 1895.

A Liberal, he was President of the Club libéral Saint-Henri between 1908 and 1911, Liberal Party organiser for the district of Montreal between 1911 and 1921, and President of the Fédération des clubs libéraux de la province de Québec in 1914.

He was elected to the Legislative Assembly of Quebec for Hochelaga in 1912 and reelected without opposition in 1916.

[1] Létourneau was appointed a judge of the Court of King's Bench of Quebec on 25 January 1922 and Chief Justice of Quebec on 9 January 1943.

Létourneau married Antonine Lanctôt on 30 June 1896.