[1] Born about 1160, his origins are unknown but he is taken to be from the same family of knightly status in or near Château-du-Loir that produced his contemporary Peter des Roches, the Bishop of Winchester.
Guillaume had participated in the defense of Le Mans in the company of such knights as William Marshal and Gerard Talbot and was with King Henry when he was forced to flee the city.
He wheeled around with William Marshal and engaged Count Richard's vanguard where he successfully charged and knocked Philip de Colombiers off of his horse.
William remained a steadfast adherent to Richard in his wars with King Philip of France from 1194–1199 and it may have been at this time that he was arranged to be married to Marguerite, the daughter and heiress of Robert de Sablé.
William, then at Le Mans, threw in his support for Arthur along with a very powerful group of Manceaux and Angevin barons, including Juhel II of Mayenne and his mother Isabella of Meulan.
She sent a force under Viscount Aimery VII of Thouars, John's newly appointed seneschal of Anjou (replacing Robert of Turnham), Hugh IX of Lusignan, and his brother Raoul I of Exoudun, count of Eu.
Eleanor's force was successful in entering the suburbs of Tours, but was driven back by King Philip II of France who had himself chosen Arthur as Richard's rightful successor.
Arthur's supporters were forced to come to terms with John, and William met with the English king at Bourg-le-Roi, a fortress of the pro-John viscounts of Beaumont-en-Maine on or about 18 September.
With this marriage came a vast landholding that included Sablé, La Suze, Briollay, Maiet, Loupelandé, Genneteil, Precigné, and the Norman manor of Agon (which was held of the lord of Mayenne).
Coinciding with a renewed French attack on upper Normandy, Arthur along with many prominent knights of France and Poitou attempted to capture Eleanor of Aquitaine as she traveled from Anjou to her chief seat at Poitiers.
Many of the prisoners captured, important Poitevins and Bretons, were grossly mistreated and some, including royal relatives like the viscount Hugh of Châtellerault and André de Chauvigny, were starved to death.
William's family had originated from the lesser aristocracy, knights from Chateau-du-Loir, a castle that was granted as a dowry property of King Richard's widow, Berengaria of Navarre.
Guillaume arranged for the exchange of his lordship of Le Mans (split with the bishop and the hereditary Manceaux seneschals, the 'Mauchien' family) for Berengaria's castle which he then became lord of.
The lasting settlement arranged in 1208 had King Philip retain authority in Touraine with the castles of Chinon, Bourgueil, Loudun, Saumur, and Langeais.
Des Roches and Dreux of Mello, constable of France conducted the attack in Touraine culminating with capture of John's last Angevin fortresses, Chinon and Loches.
As soon as the truce was up in 1208, William and Dreux de Mello (based at Loches) collected some 300 knights and launched numerous attacks on Thouars holdings in Poitou.
An army of 800 knights under the command of the seneschal, Prince Louis, Amauri I de Craon, and Henri Clement, marshal of France collected at Chinon.
The seneschal died in 1222 and his eldest daughter, Jeanne brought the Sable barony and the hereditary seneschalship to her husband, Amauri de Craon.