On June 8, 1765, fifteen days after the fire of the General Hospital of Montreal, Marguerite d’Youville signed, in the name and for the benefit of the poor of the hospital, the contract which assured her the definitive ownership of the Seigneury of Châteauguay.
With a surface area of six by nine miles, the seigneury was bounded on the north by the St. Lawrence River and Lake Saint-Louis; on the south by Beauharnois; on the east by Saut Saint-Louis and Lasalle; and on the west by the Châteauguay River. It also included Saint-Bernard Island, Île de la Paix, Île aux Pins and Île Ste-Geneviève.
Marguerite d’Youville bought these lands from Mademoiselle de Lanoue (who had been living as a lodger at the General Hospital since 1748) in order to provide resources and income for her work.
At the time of its acquisition by Mother d’Youville, there was already a wooden seigneurial house, a wooden stable, a barn and a windmill on St-Bernard Island.