Career and Patriarchate
Before his elevation, John held a sequence of offices within the church of Constantinople. Reference accounts list him as a presbyter of the city, as protekdikos, as chartophylax of the patriarchate, and as skevophylax of Hagia Sophia, indicating a long service in the administrative and liturgical life of the capital before he became patriarch.
His patriarchate fell during the reign of Emperor Constantine IV, called Pogonatos, who ruled from 668 to 685. Sources disagree on the exact length of John's tenure: one account gives roughly four years and a few months, others five years and nine months, and Theophanes records six years. He is described as a steadfast adherent of Orthodox faith and teaching amid the theological disputes of the period. He was preceded by Thomas II and succeeded by Constantine I.