[1] He participated in the Battle of Evesham in August 1265 against the royalist forces of his uncle, King Henry III of England, and his cousin, Prince Edward.

[3] In revenge for the deaths of their father and brother at Evesham, on 13 March 1271, Guy and Simon murdered Henry while he clutched the altar, begging for mercy.

The news reached England, and King Henry III dispatched a clerk of the royal household to inform the northern counties and Scotland about the excommunication.

[2] Pope Gregory X wrote a letter (29 November 1273) to King Edward from Lyons, where he was preparing for an ecumenical council, that Cardinal Riccardo Annibaldi and Cardinal Giovanni Orsini were still in Rome and had been ordered to find a secure place of imprisonment in the territories of the Church for Guy de Montfort.

Guy was stripped of his titles and took service with Charles of Anjou again, but was captured off the coast of Sicily in 1287 by the Aragonese at the Battle of the Counts.