On 21 September 1800, Lartigue was ordained a priest by Bishop Denaut at the Church of Saint-Denis on the banks of the Richelieu River, where another uncle, François Cherrier, was curé. Lartigue helped not only in the administrative affairs of the diocese, but in the pastoral duties at Longueuil, where the bishop resided as curé. Despite his poor health, he accompanied the bishop on pastoral visits; in 1803 to the Maritimes, part of the diocese, where no bishop had visited since the late 17th century.