In 1453, after the [[Fall of Constantinople]], Gennadius was taken prisoner by the Turks. Mehmed II, finding that the patriarchal chair had been vacant for some time, and wanting to use the Church to stabilize his empire, resolved to elect someone to the office, and the sultan compelled Gennadius to accept the title. Mehmed gave Gennadius both ecclesiastical and political authority, and as a result, under Gennadius, the Greek Orthodox Church became a civil as well as a religious entity.
In the spring of 1454 he was [[consecration of a bishop|consecrated]] by the [[metropolitan]] of [[Heraclea]], but, since both the Church of St. Sophia and the palace of the patriarch were now in the hands of the Ottoman Empire, he took up his residence successively in two monasteries of the city. While holding the episcopal office Gennadius drew up, apparently for the use of Mehmed, a confession or exposition of the Christian faith, which was translated into Turkish by Ahmed, judge of Beroea (and first printed by A. Brassicanus at Vienna in 1530).
Gennadius was unhappy as [[patriarch]], and tried to abdicate his position at least twice. Eventually, he found the tensions between the Greeks and the Ottomans overwhelming, and he retired to the Monastery of John the Baptist near Serrae in Macedonia, where he died in about 1473. About 100 of his alleged writings exist, the majority in manuscript and of doubtful authenticity.