Born at Constantinople, Stephen was the son of Eudokia Ingerina and, officially, Emperor Basil I.

Consequently, it is most probable that like his older brother Leo VI the Wise, Stephen was Michael's son.

Castrated by Basil I, Stephen became a monk and was designated for a career in the church since his childhood.

In 886, his brother, the new Emperor Leo VI, dismissed the Photius I of Constantinople and appointed the 19-year-old Stephen as patriarch in his stead.

[1] As patriarch, Stephen I participated in the ceremonial reburial of Michael III by Leo VI in the imperial mausoleum attached to the Church of the Holy Apostles in Constantinople.