He practised with people heavily involved in Canadian and Quebec politics: Honoré Mercier, whose daughter he married, Louis-Olivier Taillon, Raymond Préfontaine, Joseph-Émery Robidoux, Louis-Philippe Bérard and Rodolphe Lemieux.

He began his long political career by losing as a Liberal candidate in the riding of Richelieu in the 1891 federal election.

Along with his colleagues Alexander Weir and Adélard Turgeon, he resigned with a view to overthrowing Parent, who had to leave in the face of this rebellion in his own party.

In the 1908 election, faced with a surge of nationalists led by Henri Bourassa and Armand Lavergne, he ran in both Portneuf and Saint-Hyacinthe, as the law of the day allowed.

William Lyon Mackenzie King's Liberals then took power and Gouin was appointed Minister of Justice (1921-1924).

On the following 18 March, while in Parliament to prorogue a session that was running late, he suffered a heart attack and died in his office.