A courtier of Emperor Nikephoros I (r. 802–811), he survived the disastrous campaign against the Bulgars and was preferred as imperial successor over Staurakios (r. 811–811), who was severely injured.

He improved relations with the Franks, even to the point of recognising Charlemagne as an emperor – although not "of the Romans" – and requesting papal arbitration in the Moechian controversy.

Michael survived Nikephoros I's disastrous campaign against Khan Krum of Bulgaria, and was considered a more appropriate candidate for the throne than his severely injured brother-in-law Staurakios, who was proclaimed emperor by the military in the hope that he would recover.

[9] Ruling with the support of the iconodule party in the Church, Michael I diligently persecuted the iconoclasts and forced Patriarch Nicephorus I to back down in his dispute with Theodore the Stoudite, the influential abbot of the monastery of Stoudios.

Theophanes also mentions the existence of a group of heretics known as Athinganoi in Phrygia, who were most likely the successors of the Montanists, who Emperor Leo III the Isaurian (r. 717–741) attempted to convert by force, as well as of the Paulicians.

[13][14][15] There were also minor successes against the Arabs in the Anatolic Theme under its stratēgos Leo the Armenian, another figure exiled by Nikephoros and recalled by Michael.

The Bulgars nevertheless managed to capture almost all of the fortified towns on the Byzantine-Bulgarian border that were built and consolidated by Empress Irene and Emperors Constantine VI and Nikephoros in the previous decades.

[18][6] His sons were castrated to end the dynasty and were relegated to monasteries,[18] with one of them, Niketas (renamed Ignatios), eventually becoming Ecumenical Patriarch of Constantinople.

According to the iconodule Patriarch Nicephorus I of Constantinople, Leo III the Isaurian propaganda ascribed both their military successes and longevity to their iconoclasm.

The validity of iconoclasm was only confirmed by the military disasters under the iconodule emperors Michael I and his predecessor Nikephoros I (r. 802–811) as well as the success of the iconoclast Leo V the Armenian (r. 813–820) against the Bulgars.

[23] Warren Treadgold describes him as "amiable to a fault, lacking in judgment and easily led", "dithering", seeming "to have done whatever he had been told most recently".