Bermudo was elected by the palatine officials (the nobility of the royal palace) to replace Mauregatus, who had died of natural causes in 788.

He was forced to defend against an Arab-Berber invasion of Álava and Galicia and was defeated at the Battle of the Burbia River, probably the Bierzo, in 791.

Though the closest Christian sources do not name his opponents, the battle can be linked with the first major engagement of a series of aggressive campaigns launched against the Asturian kingdom in the 790s.

Bermudo abdicated his throne after his defeat, though whether volitionally (as the Chronicle of Alfonso III states, "because he was [or remembered he was] a deacon") or under duress is unknown.

Nevertheless, he was considered a generous and illustrious man in his time, "merciful and pious" in the words of the Chronicle of Albelda.