 He was the son of the civil servant Léon de Foy (1852–1942) and his wife Mathilde (née de Vos; 1860–1943).His brother Marcel de Foy (1890–1964) ended his career as President of the Court of Appeal in Brussels, and his brother Joseph de Foy (1897–1967) was an officer in the Belgian army.In 1919 Léon obtained the change of his name from "Defoy" into "de Foy" and in 1934 he acquired hereditary nobility.