The Chronicle of Alfonso III names "Alfonso the son of Peter, who was the leader of the Cantabrians and was of the royal line" when recording his arrival in Asturias. The Historia Silense records that "Petrus ex Recaredi…Gotorum principis progenie" had "duos filios…Adefonsum…et Froylam". His birth date range is estimated based on his having fought alongside his father-in-law, therefore before 737, according to the Chronicle of Alfonso III.
[Duque de Cantabria.] The Chronicle of Alfonso III states that, after the death of Fáfila (his brother-in-law, in 739), he was elected to succeed as ALFONSO I "el Católico" King of Asturias. He took advantage of instability in the Muslim Government of al-Andalus following the Berber rebellion of 741, and of a severe famine in 750, to expand the area of Christian settlements to the north of the Duero river, a list of the towns he conquered being set out in the Chronicle of Alfonso III. Ibn Idhari´s Al-Bayan Al-Moghrib records 748-753 as years of drought and famine, that in 750 “les habitants de la Galice se soulevèrent” and that many campaigns were directed against them, and that as a result of the famine “la majeure partie de la population [musulmane] émigra à Tanger, à Zawila et sur le littoral africain” departing from “la rivière de Sidona, connu sous le nom de Barbat”. King Alfonso established fortifications along the central Cordillera at Coimbra, Coria, Talavera, Madrid, Guadalajara and Medinaceli, fixing the northern frontier of the Muslim occupation for the next three centuries. On the other hand, King Alfonso was less successful along his eastern frontier, where the Muslims established settlements as far as the Pyrenees, including Pamplona, Tudela, Huesca, Girona and even Narbonne until it was captured by Pépin King of the Franks in .
The Chronicle of Alfonso III states that King Alfonso "lived as king for eighteen years and died a natural death". The Chronicon Compostellani records that “Alfonsus” reigned 19 years, one month and one day. Ibn-Khaldun states that "Alphonse fils de Pedro" died "en 142" ([4 May 759/22 Apr 760]) after reigning 18 years. The Sebastiani Chronicon records that "Adefonsus…cum uxore sua Regina Ermesinda" was buried "in territorio Cangas in Monasterio S. Mariæ". 1