In exchange, Montfort-l'Amaury was elevated to a county, and several years later, in 1230, Amaury succeeded his uncle Mathieu II of Montmorency as Constable of France.

However, the affair lasted for almost a decade: only on 11 April 1239 Amaury officially renounced his rights in England, and King Henry III recognised Simon as Earl of Leicester.

[4] In 1239 he departed for the Holy Land in the Barons' Crusade with Theobald I of Navarre, Hugh IV, Duke of Burgundy and many other prominent nobles of the realm.

King Louis IX did not go on crusade, but gave the expedition a royal character by permitting Amaury to carry the Fleur-de-lys.

He died in Otranto later the same year on his way home and was buried, at the Pope's order, in St. Peter's Basilica; his heart, according to his own wish at his death, was brought to the Abbey of Haute-Bruyère near Montfort-l'Amaury where Aubry Le Cornu, bishop of Chartres, enclosed it in an effigy.