He articled in law, was called to the Quebec bar in 1872 and became a successful criminal lawyer.

Lemieux was the head of Louis Riel's defence team, which included several other leading lawyers from eastern Canada, at his trial for high treason in 1885.

[1] The jury returned a verdict of guilty with a recommendation for mercy, but the death penalty was mandatory under the statute.

In 1892, Lemieux successfully defended Honoré Mercier against a charge of having defrauded the public treasury.

Lemieux had been reelected to the legislative assembly in both Lévis and Bonaventure in 1897, but resigned his seat when he was named judge in the Quebec Superior Court for Athabaska district.