Jean-Jacques Lartigue was born in Montreal on June 20, 1777 – the only son of a French father, Jacques Lartigue, who was a doctor that immigrated to New France in 1757, and a Canadian mother, Marie-Charlotte Cherrier, of Saint-Denis sur le Richelieu. Through his mother, Jean-Jacques Lartigue was related to three great families of the Montreal bourgeoisie; the Cherriers, the Vigers and the Papineaus; who played an important role in the community – particularly during the 1837-1838 revolt.
Intelligent and quick-witted, Jean-Jacques completed his classical studies at the age of 16 at the Collège de Montréal which was founded by the Sulpicians in 1767 following The Conquest of 1760. He then spent a full year studying English and then a further three years studying civil law. On the verge of being called to the Bar, he suddenly decided to enter the priesthood.
The Bishop of Quebec, Pierre Denaut, who lived in Longueuil where he was also a pastor, eagerly accepted Jean-Jacques' candidacy. Jean-Jacque began to study theology at the Collège de Montréal. On October 28, 1799, he was chosen by Msgr. Denaut to be his secretary. On September 21, 1800, he received priestly ordination at the age of 23 in the Saint-Denis-sur-le-Richelieu church where his uncle, François Cherrier, was the parish priest.
Having become a priest, he was appointed as vicar in Longueuil while remaining as secretary to Msgr. Denaut. This double function allowed him to acquire experience in parish ministry and knowledge of the whole “district of Montreal” by accompanying the bishop in his long pastoral visits.
After Msgr. Denaut's death on January 17, 1806 in Longueuil, where he was buried (until his remains were moved to Quebec City in 1970), Jean-Jacques realized his already professed desire to enter Saint-Sulpice. His application for admission was joyfully approved on the following February 15th. He became vicar at Notre-Dame for thirteen years (1806-1819) and was responsible for a large neighbourhood where he visited and helped many, especially the sick and the poor.
He kept himself up to date with the country's affairs. He was sympathetic to certain demands of the Canadian Party without, however, subscribing to the doctrinal radicalism and revolutionary ideas of civil disobedience and armed revolt that circulated there.
Finally, on June 29, 1819, the superior of the Séminaire de Montreal, Mr. Auguste Roux, entrusted Mr. Lartigue with a sensitive mission: to go to London and defend the rights of Saint-Sulpice which were still contested by the British authorities. There were numerous attempts at outreach to London and Paris; however, they remained unsuccessful.