Being the only son of Hugh of Brienne and Isabella de la Roche, Walter was the heir to large estates in France, the Kingdom of Naples, and the Peloponnese.

He was held in custody in the Sicilian castle of Augusta between 1287 and 1296 or 1297 to secure the payment of his father's ransom to the Aragonese admiral Roger of Lauria.

Their cousin Eschiva of Ibelin also claimed the duchy, but the High Court of Achaea passed a judgement in Walter's favor.

John II Doukas, the Greek lord of Thessaly, made an alliance against him with the Byzantine Empire and the Despotate of Epirus.

[7] His father, who was a military commander of King Charles II of Naples, fell into captivity in the Battle of the Counts on 23 June 1287.

[8] Hugh was released only after he ceded Walter as a hostage to the Aragonese admiral, Roger of Lauria, to guarantee the payment of his ransom.

[7] King Charles II ordered Hugh's southern Italian vassals to swear fealty to Walter on 27 August.

[11] They hired 300 horsemen, who were known as the "Knights of Death", and joined the army that Charles II's heir, Robert, Duke of Calabria, had mustered to invade Sicily.

[11] Before long, rumours reached the Neapolitan camp, hinting that the castellan of Gagliano Castelferrato was willing to capitulate without resistance.

[12] Charles II appointed Philip of Toucy to administer Walter's southern Italian domains during his captivity.

[6][13] Before departing for Athens, Walter appointed his father-in-law, Gaucher V de Châtillon, to administer the County of Brienne.

[14] By the time he reached Athens, John II Doukas, ruler of Thessaly, had got rid of Athenian suzerainty.

[14] The dismissed mercenaries refused to move and requested Walter to allow them to settle in the newly conquered lands as his vassals.