Flavian was a presbyter and the guardian of the sacred vessels of the great Church of Constantinople and, according to Nikephoros Kallistos Xanthopoulos, was reputed to lead a saintly life, when he was chosen to follow Proclus as Archbishop of Constantinople.
During his consecration, Roman Emperor Theodosius II was staying at Chalcedon. His eunuch Chrysaphius attempted to demand a present of gold to the Emperor but he was unsuccessful. So he plotted against the new Archbishop by supporting Eutyches in his dispute with Flavian.
Flavian presided at a council of forty bishops at Constantinople on November 8, 448. He wanted, to resolve a disagreement between the bishop of Sardis and two bishops of his province. Eusebius, bishop of Dorylaeum, presented an indictment against Eutyches. Flavian’s speech from this council still exists. Eventually, the council deposed Eutyches.
Eutyches protested against this verdict and received the support of Dioscorus I of Alexandria. The Emperor called another meeting, The Council to Ephesus. At this meeting on August 8, 449, Flavian was beaten during the sessions of this council by impudent monks led by a certain Barsumas. Flavian was then deposed, exiled, and the council reinstated Eutyches.
Flavian died on August 11, 449, in Asia Minor and was buried obscurely.