Patriarch of Antioch, and a friend of Theodoret. He was nephew of John I, Patriarch of Antioch (428-442), brought up under Euthymius the famous hermit of Palestine. He was ordained deacon by Juvenal of Jerusalem on his visit to the Laura of Euthymus in 429. On the death of his uncle John in 441/2, he usurped the patriarchate and was installed as a patriarch of Antioch by the pagan governor of Antioch, in violation of canon law, without the official sanction of the Holy Synod of Antioch or election, which is untraditional way of the church, and at once ranked as the Patriarch of the Holy See of Antioch.

In 445 he summoned a synod of Syriac bishops which confirmed the deposition of Athanasius of Perrha. In 447 he consecrated Irenaeus to the see of Tyre (Theodoret, Epistle 110); but emperor Theodosius II, commanded that the appointment should be annulled on the grounds that Irenaeus was both a digamus and a supporter of Nestorianism. He installed Uron the bishop on the see of Homs. He defended Hiba (Ibas) bishop of Edessa, against charges of promulgating Nestorian doctrines, and summoned a council at Antioch (448) which decided in favor of Hiba and deposed his accusers. Domnus's sentence, though revoked by Flavian, Patriarch of Constantinople, was confirmed by three episcopal commissioners to whom he and the emperor Theodosius had committed the matter. As a result of the Council of Ephesus II which held on August 8, 449, and presided by Dioscorus, he was depose. The council appointed Maximus II, as his successor on the Holy See of Antioch.

Obtained permission to assign Domnus a pension from the revenues of the church, and on his recall from exile Domnus returned to the monastic home of his youth, ending his days in the Laura of St. Euthymius, where in 452, according to Theophanes, he afforded a refuge to Juvenal of Jerusalem when he was driven from his See.