Guy de Montfort was the Count of Bigorre from 6 November 1216 to 1220 in right of his wife, Petronilla. He was a son of Simon IV de Montfort

Simon de Montfort, 5th Earl of Leicester

Simon IV de Montfort, Seigneur de Montfort-l'Amaury, 5th Earl of Leicester , also known as Simon de Montfort the elder, was a French nobleman who took part in the Fourth Crusade and was a prominent leader of the Albigensian Crusade...

 and Alice of Montmorency.

Guy joined his father on the Albigensian Crusade

Albigensian Crusade

The Albigensian Crusade or Cathar Crusade was a 20-year military campaign initiated by the Catholic Church to eliminate Catharism in Languedoc...

 while still quite young. Late in 1216, he married Petronilla, the heiress to Bigorre and Marsan through her mother Stephanie, and a daughter of Bernard IV of Comminges.

He fought at his father's side at the siege of Toulouse in 1218, but his father died: crushed by the projectile of a siege engine

Siege engine

A siege engine is a device that is designed to break or circumvent city walls and other fortifications in siege warfare. Some have been operated close to the fortifications, while others have been used to attack from a distance. From antiquity, siege engines were constructed largely of wood and...

. Amaury VI de Montfort inherited his father's command, but not his strategic vision. The Occitan lords rebelled against him and Guy was killed in a conflict at Castelnaudary

Castelnaudary

Castelnaudary is a commune in the Aude department in the Languedoc-Roussillon region in south France. It is in the former province of the Lauragais and famous for cassoulet of which it claims to be the world capital, and of which it is a major producer....

 in 1220, either on 4 April or in July.

Guy left a daughter, Alice, who succeeded Petronilla. He also left a daughter named Pernelle, who married Raoul de la Roche-Tesson.