He was, according to the Genealogica Comitum Guasconiæ, the eldest son of Arnold II of Astarac and Talesa.
In 1023, in his first recorded act as count, William received the monastery of Pessan from his younger brother Otto (or Oddo), a deacon, and granted it to the abbey of Simorre, of which Otto became abbot.
In 1025 William consented to Otto's restoration of the monastery of Saramon.
In 1034 William placed the abbey of Sainte-Dode, founded earlier by Otto, under the authority of Simorre.
In that same year William married a relative within the degree prohibited by the church.