The friend and disciple of Saint John Chrysostom, Proclus became secretary to Archbishop Atticus of Constantinople (406–425). who ordained him deacon and priest. Atticus' successor, Sisinnius I (426–427), consecrated him Bishop of Cyzicus, but the people there refused to receive him, and he remained at Constantinople. On the death of Sisinnius, the famous Nestorius succeeded as Archbishop of Constantinople (428–431), and early in 429, on a festival of the Theotokos (Virgin Mary), Proclus preached his celebrated sermon on the Incarnation, which was later inserted in the beginning of the Acts of the Council of Ephesus.
When Archbishop Maximianus (431–434) died on Great and Holy Thursday, Proclus was immediately enthroned by the permission of the Emperor Theodosius II and the bishops gathered at Constantinople. His first care was the funeral of his predecessor, and he then sent to both Patriarchs Cyril of Alexandria and John of Antioch the usual synodical letters announcing his appointment, both of whom approved of it.
In 436 the bishops of Armenia consulted Proclus upon certain doctrines prevalent in their country and attributed to Theodore of Mopsuestia, asking for their condemnation. Proclus replied the next year in the celebrated letter known as the
Tome to the Armenians, which he sent to the Eastern bishops, asking them to sign it and to join in condemning the doctrines arraigned by the Armenians. They approved of the letters, but from admiration of Theodore hesitated to condemn the doctrines attributed to him. Proclus replied that while he desired the extracts subjoined to his Tome to be condemned, he had not attributed them to Theodore or any individual, not desiring the condemnation of any person.
A rescript from Theodosius procured by Proclus, declaring his wish that all should live in peace and that no imputation should be made against anyone who died in communion with the church, appeased the storm. The whole affair showed conspicuously the moderation and tact of Proclus. In 438, he transferred the relics of his old master, Saint John Chrysostom, from Comana back to Constantinople, where he interred them with great honour in the Church of the Twelve apostles. This action reconciled to the church those of Saint John's adherents who had separated themselves in consequence of the deposition as Archbishop which they regarded as having been unjust.
In 439, at the request of a deputation from Caesarea in Cappadocia, Proclus selected as their new bishop
Thalassius, who was about to be appointed praetorian prefect of the East.
In the time of Proclus the Trisagion came into use. The occasion is said to have been a time when violent earthquakes lasted for four months at Constantinople, so that the people were obliged to leave the city and encamp in the fields.
Proclus died most probably in July, 446. He appears to have been wise, moderate, and conciliatory, desirous, while strictly adhering to Orthodoxy himself, to win over those who differed from him by persuasion rather than force.