Orderic Vitalis names “Guillelmum” as the son of “Rodbertum Belesmensem” and his wife “filiam Guidonis Pontivi comitis Agnetem”.
He succeeded his father [4 Mar 1106/1110] as Comte de Ponthieu. "Wilelmus comes Pontivi" confirmed the donation of the church of Saint-Léonard de Bellême to Marmoutier by charter dated to [1112/14]. Henry I King of England restored Comte Guillaume to his father's lands in Normandy in Jun 1119. He resigned Ponthieu [before 1129] in favour of his son Guy: Guillaume of Jumièges records that ”Willelmum Talavatium” married “Ala...quæ fuerat antea uxor ducis Burgundiæ” [incorrect], and had “duos filios et totidem filias” of whom “filiorum primogenitus Wido” was made “patre vivente...comes Pontivi”. "Gulielmus comes Pontivorum" donated property to the abbey of Saint-Sauveur-le-Vicomte for the souls of "his predecessors earl Roger and Mabel his wife, count Guy and Adda his wife, his father Robert de Belesmo and his mother Agnes and…Ela his wife and of their sons two of them called Robert and two William and two Enguerrand and Mabel his daughter" by charter dated 1127. Robert of Torigny records that Henry II King of England granted "castrum Alenceium et Rocam Mabiriæ" to "Willermus Talavacius comes Sagiensis et filius eius Johannes et iterum Johannes nepos eius filius Guidonis primogeniti sui comitis Pontivi" in 1166.
Robert of Torigny records the death in 1171 of "Guillermus Talavercius comes Pontivi" and the succession of "Johannes nepos suus…ex Guidone primogenito suo". The Obituaire of Saint-Martin de Séez records the death 30 Jun of "Guillelmus Pontivorum comes". The necrology of the monastery of Ouche records the death "30 Jun" of "Willelmus comes Talevat". 2
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