[citation needed] When Timothy Aelurus, who had been expelled in 460 and returned in 475, died in 477, his followers elected Mongus to succeed him.
However, the Byzantine Emperor Zeno brought Timothy Salophakiolos, a Chalcedonian who had supplanted Aelurus before in 460, back to Alexandria and sentenced Mongus to death.
He held a synod to condemn Chalcedon, and desecrated the tombs of his two Chalcedonian predecessors Proterios and Timothy Salophakiolos.
[citation needed] When Acacius died in 489, Mongus encouraged his successor Fravitta to maintain the schism with Rome.
A pretended correspondence between him and Acacius (in Coptic) is proved to be spurious by Amélineau in the "Memoires publiés par les membres de la mission archéologique française au Caire", IV (Paris, 1888), 196–228.