Joannes (surnamed "Jejunator", sometimes also "Cappadox") was born at Constantinople of artisan parents, and worked as a goldsmith. Under the Patriarch John III (565–577) he was deacon at the Hagia Sophia church; then he became sakellarios (an official who acts as patriarchal vicar for monasteries). He was famous for his ascetical life and called "the Faster". Under Eutychius I (577–582) he became an important person among the clergy of the city. At Eutychius's death he was made patriarch by the Emperor Tiberius II (578–582).