He was the son of Simon de Montfort (d. 1188), lord of Montfort l'Amaury in France near Paris, and Amicia de Beaumont, daughter of Robert de Beaumont, 3rd Earl of Leicester - the de Montfort line itself descends from the Counts of Flanders. He succeeded his father as Baron de Montfort in 1181; in 1190 he married Alix de Montmorency, the daughter of Bouchard III de Montmorency. She shared his piety and would accompany him on his campaigns. In 1191 his brother, Guy, left on the Third Crusade in the retinue of King Philip II of France.