Nicephorus was the son of an imperial Byzantine secretary who had been deposed, beaten, and exiled under the first iconoclast emperor Constantine V (754–775) for refusing to subscribe to the iconoclast canons — placing the son in the first-generation iconodule-confessor tradition.
Despite this dangerous family background, his learning and political skills brought him to high imperial-bureaucratic office under the iconodule empress Irene (797–802), where he played a role in the iconodule consolidation that produced the Second Council of Nicaea (787) under his eventual predecessor Tarasios.
Around 803 he withdrew from the imperial court and founded a private monastery on the Bosphorus. On Tarasios's death in 806, he was elected — over Theodore the Studite's monastic-party objection — Patriarch of Constantinople by Emperor Nikephoros I.
When the second iconoclast crisis broke out under the new emperor Leo V the Armenian (813), Nicephorus stood firm: refusing to remove the icons from Hagia Sophia, refusing to sign the new iconoclast formula, and confronting the emperor directly. In March 815, Leo V deposed and exiled him to the monastery of St. Theodore at Chrysopolis on the Asian side of the Bosphorus.
He spent thirteen years in monastic-imposed exile (815–828), composing his theological refutations of iconoclasm — the three Antirrhetici (Refutations) against the iconoclast claims, the Apology and the Greater and Lesser Refutations against the iconoclast Council of Hieria of 754. He died in exile on June 5, 828; his relics were translated to Hagia Sophia on March 13, 847 by Empress Theodora and her iconodule restoration — the date kept in the Latin Catholic calendar.