Born Guy Philippe Henri Lannes de Montebello in Paris in 1936 to a family descended from the Napoleonic aristocracy, de Montebello was the second of four sons. His father, Marquis André Roger Lannes de Montebello (Biarritz, July 6, 1908 - New York City, December 2, 1986), was a portrait painter, art critic, and a member of the French Resistance during World War II. His mother, Germaine Wiener de Croisset (born in Paris, October 26, 1913 and married in Paris, November 30, 1933), was a descendant of the Marquis de Sade, a daughter of the playwright Francis de Croisset, and a half-sister of the arts patron Marie-Laure de Noailles. One of de Montebello's great-great-great-grandfathers was Jean Lannes, Duke of Montebello and Marshal of France.