Little of Demophilus' early life is known. He was born in Thessalonica and was Bishop of Berea before being elected Bishop of Constantinople in 370 by the Arian party of bishops. Earlier, the Orthodox party in the religiously divided city elected Evagrius as their bishop, but he and Eustathius, who had consecrated Evagrius to the Constantinople see, were both banished by Emperor Valens, a supporter of Demophilus and the Arian cause. The followers of Evagrius and Eustathius were persecuted severely.
After his ascension as patriarch, Demophilus was unsuccessful in obtaining the election of an Arian as bishop of Cyzicus, a position left vacant after Eucomius was banished. The people of Cyzicus refused to recognize Demophilus, as well as Dorothesus who accompanied him, until after they had anathematized Aetius, Eucomius, and their followers. Demophilus was then allowed by the people to consecrate a bishop chosen by them. After his consecration the new bishop of Cyzicus taught forthrightly the faith of the Nicene Creed.
In 380, after Theodosius I had ascended to the imperial throne, he offered Demophilus a confirmation to the patriarchal see if he would agree to the Nicene Creed. After Demophilus refused the offer, Theodosius immediately directed Demophilus to surrender all his churches.  Whereupon Demophilus retired with his followers to a church outside Constantinople thus leaving the churches in Constantinople, and adjacent cities, in the hands of the Orthodox for the first time after forty years of control by the Arians.
Demophilus reposed in 386.
|Bishop of Berea|
|Bishop of Constantinople|