Background
He was the only son of William I and inherited the entire county of Astarac, which had been reduced in area by partitioning among heirs in earlier generations. His eldest, William, is mentioned in a document of about 1075, but died before his father.
Career
Sancho had at least three sons. The only information provided on Sancho by the Genealogia comitum Guasconiae (Genealogy of the Counts of Gascony) in the archives of the cathedral of Sainte-Marie d"Auch is that "William begat Sancho Sancho begat Bernard." Sancho"s wife is mentioned in a document from about 1075, but is not named. Little is known of Sancho"s rule other than his re-organisation of the monasteries of Astarac.
Around 1050 he granted some rights he possessed in Saint-Maur, as well as the monastery there, to the abbey of Saint-Pé-de-Bigorre.
Around 1075 he placed the monastery of Sainte-Dode, which had been founded by his father, under the authority of Simorre. About the same time he also donated the monastery of Saramon to that of Sorèze.
A certain Arnaud of Astarac joined Sancho on this occasion and donated Salvetat de Sainte-Foye to the Holy Sepulchre. His parentage is not mentioned, but he was probably a relative of Sancho.
The charter of donation was witnessed by Raymond II de Pardiac, who was the archbishop of Auch from 1096 to 1118/9.
Nicolas Guinaudeau dates it to around 1096, but Helen Nicholson places it after the First Crusade (1096-1099), when the Holy Sepulchre came under Christian control.