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.

Denault's death in 1806 gave Lartigue the freedom to become a member of the Society of Saint-Sulpice, into which he was received in February of that year, seeking a more contemplative and intellectual life.
He joined the Sulpician community of the Seminary, the first native Canadian to enter the Society.
He was assigned to help in the Parish of Notre-Dame which was attached to the Seminary.
That same year, Denault's successor, Joseph-Octave Plessis, called upon his legal knowledge to challenge an effort by Attorney General Jonathan Sewell to challenge the legal standing of Catholic parishes created since the British conquest of New France.

In 1819 the Seminary faced a legal challenge from the British governor of the province to its holding of various seigneuries in Quebec, which were its primary means of support.
Lartigue was entrusted with the superior of the Seminary with a mission to London to present their case directly to the government in England.
The superior chose Lartigue as being particularly qualified for the mission because of his knowledge of the law and his mastery of English.
For this he was to accompany Bishop Plessis, who was traveling there to secure letters patent for the establishment of a new seminary and for permission to divide the Diocese of Quebec, which was proving unmanageable.
They sailed for London on 3 July 1819 on the George Symes, arriving on 14 August.

Lartigue spent the following two months meeting with various officials of the British government, even the Vicar Apostolic in London, who was the chief authority for the Catholic Church in the United Kingdom.
He seemed to be making no progress in his cause, so in late October he traveled to Paris where he spent a month trying to find some way of securing the support of the French government to intercede for them.
He returned to Canada, believing his mission a failure.
Nothing, however, came of the Canadian governor's efforts to seize the Seminary's holdings.
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.
Denault's death in 1806 gave Lartigue the freedom to become a member of the Society of Saint-Sulpice, into which he was received in February of that year, seeking a more contemplative and intellectual life. He joined the Sulpician community of the Seminary, the first native Canadian to enter the Society. He was assigned to help in the Parish of Notre-Dame which was attached to the Seminary. That same year, Denault's successor, Joseph-Octave Plessis, called upon his legal knowledge to challenge an effort by Attorney General Jonathan Sewell to challenge the legal standing of Catholic parishes created since the British conquest of New France.
In 1819 the Seminary faced a legal challenge from the British governor of the province to its holding of various seigneuries in Quebec, which were its primary means of support. Lartigue was entrusted with the superior of the Seminary with a mission to London to present their case directly to the government in England. The superior chose Lartigue as being particularly qualified for the mission because of his knowledge of the law and his mastery of English. For this he was to accompany Bishop Plessis, who was traveling there to secure letters patent for the establishment of a new seminary and for permission to divide the Diocese of Quebec, which was proving unmanageable. They sailed for London on 3 July 1819 on the George Symes, arriving on 14 August.
Lartigue spent the following two months meeting with various officials of the British government, even the Vicar Apostolic in London, who was the chief authority for the Catholic Church in the United Kingdom. He seemed to be making no progress in his cause, so in late October he traveled to Paris where he spent a month trying to find some way of securing the support of the French government to intercede for them. He returned to Canada, believing his mission a failure. Nothing, however, came of the Canadian governor's efforts to seize the Seminary's holdings.
Jean-Jacques Lartigue