 He was born in Yamachiche, Lower Canada in 1823, the son of François Lesieur Desaulniers, who represented Saint-Maurice in the legislative assemblies for Lower Canada and the Province of Canada.He studied at the Séminaire de Nicolet, then went on to study medicine, first at Trois-Rivières and then at Harvard University, graduating in 1846.He returned to practice in Yamachiche.