Elected in 1261, he received consecration at Rome from Pope Urban IV in 1263.
Amanieu was the third son of Roger, viscount of Fezensaguet, and Pucelle d'Albret, and thus a descendant of the Counts of Armagnac and the Sires of Albret.
Amanieu's eldest brother was Geraud VI of Armagnac.
In 1278 Geraud and Amanieu signed a treaty sharing jurisdiction in Barran, where the bishop had his summer residence, between the Church and the County.
Pope Gregory X dissolved the order in 1273 and its possessions ended up largely in the hands of the Order of Santiago and the church of Auch, though Les Feuillants Abbey laid claim to some.