Gautier or Walter V of Brienne was born in Brienne-le-Château, Aube, Champagne, France. He was the son of Hugh, Count of Brienne and Lecce, and Isabella de la Roche, daughter of Guy I de la Roche, Duke of Athens. He was the heir of the Brienne claim to the Kingdom of Jerusalem and of Cyprus, as well as to Taranto and Sicily.
Walter spent his youth as a hostage in Sicily, in the castle of Agosta. On the death of his father Hugh in 1296, Walter inherited the titles of Count of Brienne, Conversano and Lecce.
The death of his mother's first cousin, Guy II of la Roche, in 1308 brought him the Duchy of Athens. There he found himself hard pressed by the Despot of Epirus, the Emperor Andronikos II Palaiologos and the Lord of Vlachia (Thessaly), John II Doukas. In 1310, he hired the Catalan Company, then ravaging the Byzantine Empire, to fight the Byzantine Greeks encroaching on his territory.
After the Company had successfully reduced his enemies, he attempted to expel the Company from Athens with their pay in arrears. The Company refusing this, Walter marched out with a strong force of French knights from Athens, the Morea and Naples and Greek foot from Athens. Walter's army met the Catalans at the Battle of Halmyros (located either near Halmyros in Thessaly or near Orchomenos in Boeotia) on 15 March 1311. The Catalans won a devastating victory, killing Walter and almost all of his cavalry, and seizing his Duchy of Athens, excepting only the Lordship of Argos and Nauplia.
The Catalan Company nominated one of the surviving knights, Roger Deslaur, as their leader and new Duke of Athens by conquest.