He was born in Oviedo in 759 or 760. He was put under the guardianship of his aunt Adosinda after his father's death, but one tradition relates his being put in the monastery of Samos. He was the governor of the palace during the reign of Adosinda's husband Silo. On Silo's death, he was elected king by Adosinda's allies, but the magnates raised his uncle Mauregatus to the throne instead. Alfonso fled to Álava to live with his maternal relatives.
Mauregatus was succeeded by Bermudo, Alfonso's cousin, who abdicated after his defeat at Burbia. Alfonso was subsequently elected king on 14 September 791. The events of his reign are in reality almost unknown. Poets of a later generation invented the story of the secret marriage of his sister Ximena with Sancho, count of Saldana, and the feats of their son Bernardo del Carpio. Bernardo is the hero of a cantar de gesta (chanson de geste) written to please the anarchical spirit of the nobles.
What is known is that he maintained contact with the court of Charlemagne. He sent delegations to Aachen in 796, 797, and 798, but we do not know the purposes. They may have dealt with the security of his kingdom from the ongoing attacks of the Ibn Mugait brothers. On the other hand, they may have been related to the adoptionist controversy which had brought Bermudo's kingdom into Charlemagne's view.
Militarily, Alfonso did much to secure his own realm against the Moors. He took Lisbon in 798. He defeated the Moslems at Narón and Anceo (825) and, thanks to these victories, began the repopulation of parts of Galicia, León, and Castile.
Alfonso also moved the capital from Pravia, where Silo had located it, to Oviedo, the city of his father's founding and his birth. There he constructed churches and a palace. He built San Tirso, where he is buried, and Santullano, on the outskirts. The Crónica Sebastianense records his death in 842, saying: