In 1776, James McGill married Marie-Charlotte (1747-1818), the widow of Joseph-Amable Trottier Desrivières (1732-1771). Educated at the Ursuline Convent, Quebec, she was the daughter of Guillaume Guillimin (1713-1750), Member of the Sovereign Council of New France and Judge of the Courts of Admiralty. Her mother, Marie-Geneviève Foucault, was the daughter of François Foucault (1690-1766), the owner of one of the largest seigneuries in New France and a member of the Sovereign Council. McGill had previously adopted Charlotte, the youngest daughter of his deceased friend John Porteous, and through his marriage (after the death of their uncle and legal guardian) he became the step-father of two sons,