As general of finances for the king, he was governor of several royal castles and accompanied Louis XII during the Italian War of 1494–1498.

Guillaume, born in 1453, was the son of Jean II de Montmorency and his second wife Marguerite d'Orgemont.

In 1484, Guillaume's maternal uncle Pierre III d'Orgemont, who died without children, bequeathed him the lordship of Chantilly and rebuilt the chapel of the castle there in 1507.

[3] King Louis XII appointed him superintendent of waters and forests and gamekeeper in Touraine, as well as captain of castle of Saint-Germain-en-Laye.

Guillaume was appointed captain of Château de Vincennes and made a knight of the Order of Saint Michael.