On May 13, 1836, Pope Gregory XVI signed the paper for the designation of the new diocese and the brief proposing Jean-Jacques Lartigue to the seat of Montreal. Before this final act, London, in an unanticipated move, accepted the new bishop.

Msgr. Lartigue took charge of his episcopal chair on September 8, 1836 with general enthusiasm. The superior of Saint-Sulpice gave an unequivocal welcome to the first bishop of Montreal in the new church of Notre-Dame (1829).

From 1836 to 1840, great harmony reigned. The “district of Montreal” finally became the diocese of Montreal; an immense territory – a square cut out in the south of Quebec, crossed by the St. Lawrence River, bounded to the north by the villages of Berthier and Sorel, to the south by the Native mission of Saint-Régis, to the east by a line beyond the Richelieu, and to the west by a line beyond Saint-Jérôme. A population of nearly 300,000 faithful lived within it amidst about 100 parishes and missions.