Nicephorus was a native of Seleucia in Pisidia, who was raised by the empress Irene to the office of logothetes or lord high treasurer. With the help of the patricians and eunuchs he contrived to dethrone and exile Irene, and to be elected emperor in her stead. His sovereignty was endangered by Bardanes, one of his ablest generals, who revolted and received support from other commanders, notably the later emperors Leo the Armenian and Michael the Amorian.
But Nicephorus gained over the latter two, and by inducing the rebel army to disperse achieved the submission of Bardanes, who was relegated to a monastery. A conspiracy headed by the patrician Arsaber had a similar issue. Nicephorus, who needed large sums to strengthen his military force, set himself with great energy to increase the empire's revenue. By his rigorous imposts he alienated the favour of his subjects.
He vigorously asserted imperial authority over the church. This policy and his appointment of St. Nicephorus to the patriarchate of Constantinople precipitated a conflict with Theodore of Studium, whom he exiled in 809.
In 803 and 810 he made a treaty with Charlemagne, by which the limits of the two empires were amicably fixed. Venice, Istria, the Dalmatian coast and South Italy were assigned to the East, while Rome, Ravenna and the Pentapolis were included in the Western realm.
By withholding the tribute which Irene had agreed to pay to Harun al-Rashid, Nicephorus committed himself to a war with the Saracens. Compelled by Bardanes's disloyalty to take the field himself, he sustained a severe defeat at Crasus in Phrygia (805), and the subsequent inroads of the enemy into Asia Minor induced him to make peace on condition of paying a yearly contribution of 30,000 gold pieces.
With the death of Harun in 809, Nicephorus was left free to deal with the Bulgarian king, Krum, who was harassing his northern frontiers. In 811 Nicephorus invaded Bulgaria and this campaign drove Krum to ask for terms, but at the Battle of Pliska on July 26, Krum surprised and slew Nicephorus along with a large portion of the Byzantine army. Krum is said to have made a drinking-cup of Nicephorus' skull.
Nicephorus was succeeded briefly by his son Stauracius and then by his son-in-law, Michael I.