He was born in Crete, where he was father superior. He served as chief of the Church of St. George in Istanbul. Later he was elected bishop of Ierissos and Mount Athos, and in 1767 metropolitan bishop of Thessaloniki. He was elected Patriarch on 11 April 1769, at a time when Christians were persecuted after the withdrawal of the Russian forces following the Orlov Revolt. Many Christians had helped the Russians, following the exhortations of the former Patriarch Seraphim II and other clerics. Theodosius made efforts to save the monasteries of Mount Athos from demolition, he freed prisoners, he supported schools and monasteries and, with cooperation with the Patriarch of JerusalemSophronius V, he managed to keep the Holy Land under the jurisdiction of the Eastern Orthodox Church. Lastly, he tried to give a conciliatory solution[1] to the issue of the Kollyvades of Mount Athos.
He was forced to resign on 16 November 1773, following the machinations of the Metropolitan of Prussa, Meletius. He retired to the Kamariotissa Monastery in Heybeliada. In 1776, having completely lost his sight, he returned to Istanbul, where he resided until his death.
Διονύσιος Δ. Βαλαής (D. D. Valais), Ο Μητροπολίτης Θεσσαλονίκης Θεοδόσιος Β΄ (Theodosius II of Constantinople) (1762-1769), Επιστημονική Επετηρίδα της Θεολογικής Σχολής του Α.Π.Θ., Νέα Σειρά, Τμήμα Θεολογίας, Τιμητικό Αφιέρωμα στον ομότιμο καθηγητή Ιωάννη Χρ. Ταρνανίδη (Scientific Annals of the Theological School of the Aristotle University of Thessaloniki, New Series, Theology Department, Honorary Tribute to Professor Emeritus J. C. Tarnanidis), 17 (2007-8), p. 15-46