Morny was born in Switzerland, and was the extra-marital son of Hortense de Beauharnais (the wife of Louis Bonaparte and queen of Holland) and Charles Joseph, Comte de Flahaut, making him half-brother of Emperor Napoleon III and grandson of Talleyrand. His birth was duly registered in a misleading certificate, which made him the legitimate son of Auguste Jean Hyacinthe Demorny, and born in Paris on 23 October 1811, and described as a landowner of St. Domingo. M. Demorny was in fact an officer in the Prussian army and a native of St. Domingo, though he owned no land there or elsewhere.