He had been appointed by the Council of Chalcedon to replace the deposed Dioscorus.

Because the church of Alexandria was largely anti-Chalcedonian, the deposition of Dioscorus, an anti-Chalcedonian, from the Patriarchate, and the elevation of Proterius, a Chalcedonian, to it, was violently opposed.

Finally in 457 the anti-Chalcedonian party in Alexandria elected Timothy Aelurus as Patriarch of Alexandria, in opposition to Proterius, who was either subsequently martyred by a Coptic mob (Evagrius Scholasticus, 2, 8 [1]) or murdered by the Byzantine garrison in Alexandria (Zachariah of Mitylene, 4,1-3[4]).

The murder was commented in several letters by groups of bishops from various Roman provinces (e.g. Galatia Prima) or larger regions to Byzantine emperor Leo I the Thracian (457–474).

(Coptic Orthodox Church Network, Popes Chronology [2]).