Arriving at Antioch AD 463, Peter greatly desired the patriarchal throne, then filled by Martyrius. He quickly befriended the populace, with whom he raised suspicions against Martyrius as a concealed Nestorian, thus causing Martyrius' tumultuous expulsion and his own election to the throne. Theodorus Lector dates this to 469 or 470. When established as patriarch, Peter at once declared himself openly against the Council of Chalcedon, and added to the Trisagion the words "Who wast crucified for us," which he imposed as a test upon all in his patriarchate, anathematizing those who declined to accept it. According to the Synodicon, he summoned a council at Antioch to give synodical authority to this novel clause (Labbe, iv. 1009). The deposed Martyrius went to Constantinople to complain to the emperor Leo, by whom, through the influence of the patriarch Gennadius, he was courteously received; a council of bishops found in his favor, and his restoration was decreed (Theodorus Lector p. 554). But despite the imperial authority, Peter's personal influence, supported by the favour of Zeno, was so great in Antioch that Martyrius's position was rendered intolerable and he soon left Antioch, abandoning his throne again to the intruder. Leo was naturally indignant at this audacious disregard of his commands, and he despatched an imperial decree for the deposition of Peter and his banishment to the Oasis (Labbe, iv. 1082).
According to Theodorus Lector, Peter fled, and Julian was unanimously elected bishop in his place (471), holding the see until Peter's third restoration by Basiliscus in 476 (Theophanes p. 99). During the interval Peter dwelt at Constantinople, in retirement in the monastery of the Acoimetae, allowed to reside there in return for a pledge that he would not create further disturbances (Theophanes p.104). During the short reign of the usurper Basiliscus (Oct. 475-June 477) the fortunes of Peter revived. Under the influence of his wife, Basiliscus declared for the Monophysites, recalled Timothy Aelurus, Patriarch of Alexandria, from exile, and by his persuasion issued an encyclical letter to the bishops calling them to anathematize the decrees of Chalcedon (Evagr. H. E. iii. 4). Peter gladly complied, and was rewarded by a third restoration to the see of Antioch, 476 (ib. 5). Julian was deposed, dying not long after.
On his restoration Peter enforced the addition to the Trisagion, and behaved with great violence to the orthodox party, crushing all opposition by an appeal to the mob, whom he had gained control over. Once established on the patriarchal throne, he was not slow to stretch its privileges to the widest extent, ordaining bishops and metropolitans for all Syria. The fall of Basiliscus brought the ruin of all who had supported him and been promoted by him, and Peter was one of the first to fall. In 485 for the last time Peter was replaced on his throne by Zeno on his signing the Henoticon (Theophanes p.115; Theodorus Lector p. 569; Evagr. H. E. iii. 16). He at once resumed his career of violence, expelling orthodox bishops who refused to sign the Henoticon and performing uncanonical ordinations, especially that of the notorious Xenaias (Philoxenus) to the see of Hierapolis (Theophanes p.115). He was condemned and anathematized by a synod of 42 Western bishops at Rome 485, and excommunicated. He retained, however, the patriarchate at Antioch till his death in 488 (or according to Theophanes, 490 or 491). One of his last acts was the unsuccessful revival of the claim of the see of Antioch to the obedience of Cyprus as part of the patriarchate, which the Council of Ephesus had removed from Antioch's supervision in 431.