Lartigue was born to a noted Montreal family, the only son of Jacques Larthigue, a surgeon, and Marie-Charlotte Cherrier.

He attended the Collège Saint-Raphaël (later the Petit Séminaire de Montréal), followed by two years at an English school run by the Sulpicians, receiving a solid education.

He then clerked for three years with a Montreal law firm where he developed a lifelong interest in the politics of Lower Canada.

He soon received minor orders and later the diaconate from Bishop Pierre Denaut of Quebec and taught at his Saint-Raphaël, while he studied for the priesthood under the Sulpicians.

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.

[1] 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.

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.

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.

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.

Even apart from the division over the role of the papacy, the Superior of the Seminary apparently feared the loss of power by their community to a prelate of the Church, even one who had been one of their own.

Thus the school became a center of ultramontanism decades before this position was proclaimed an essential element of Catholic belief at the First Vatican Council.

For two days, more than twelve hundred "Patriotes" marched in front of Saint-Jacques cathedral to protest the Bishop's directive.