He was the son of Gregory III and brother of Sergius II, whom he blinded and deposed in order to seize the throne while he was already bishop.
In this usurpation, Athanasius was originally supported (financially) by Pope John VIII, who desired to break the Neapolitan friendship with the Saracens.
He and Guaimar fought an indecisive war while the latter was preoccupied with the Saracen menace Athanasius was ignoring.
In 886, Athanasius, since released from excommunication, was allied with the Saracens again and received a threat from Pope Stephen V of a blockade of Naples.
He was a hellenophile who worked to preserve many Greek manuscripts and maintain good relations with Byzantium.