He became a farmer and merchant and was also a shareholder and administrator for the Vaudreuil Railway Company. Mongenais helped found the Collège Bourget at Rigaud. In 1848, he was elected to the Legislative Assembly for Vaudreuil; he was reelected in 1851, 1854 and 1861.
He transitioned to farming, commerce, and became a shareholder and administrator of the Vaudreuil Railway Company. Mongenais played a role in establishing the Collège Bourget at Rigaud. In 1848, he was elected to the Legislative Assembly for Vaudreuil; he was reelected in 1851, 1854, and 1861.