Theophanes was born into a senior Byzantine military family and married Megalo, the daughter of a fellow nobleman. After three years they agreed by mutual vow to part for monastic life; she entered a convent on Prinkipo, he founded the great monastery of Megas Agros on the eastern slope of Mount Sigriane in Bithynia.
His Chronographia — covering the years 284 to 813 in continuous annal form — is the principal Greek source for the 7th and 8th centuries, including extensive coverage of the rise of Islam, the iconoclast controversy, and the early Carolingians. Latin and Greek scholars have depended on it for over twelve hundred years.
Refusing to sign the renewed iconoclast decrees of Emperor Leo V the Armenian in 815, he was imprisoned for two years, tortured, and finally exiled to the island of Samothrace, where he died in 818 of dropsy and a stomach ulcer. He is honored as a confessor of the faith and one of the principal historians of the medieval East.