He continued the policies of his father, Gregory III, and grandfather, Sergius I.
He maintained good relations with the Franks or the Byzantines only as it suited Neapolitan interests.
It is written that Sergius made Naples "into another Palermo, another Africa," by his friendly relations with the Aghlabids.
He also earned the opposition of his uncle, Bishop Athanasius I and exiled him to an island.
Sergius was blinded and deposed by his brother Athanasius II, Bishop of Naples, who delivered him to Rome.