In 696, Tiberius was part of an army sent by Byzantine Emperor Leontius to retake the North African city of Carthage, which had been captured by the Arab Umayyads.

[7] The Byzantinist Walter Kaegi states that Tiberius had won victories over the Slavs in the Balkans during his early military career, which granted him a degree of popularity.

A group of officers who feared Leontius's wrath for failing to recapture Carthage killed John, and declared Apsimar emperor.

[10] He gathered a fleet and allied himself with the Greens (one of the Hippodrome sports and political factions), before sailing for Constantinople, which was enduring an outbreak of the bubonic plague.

[5][12][14] According to the 12th-century chronicler Michael the Syrian, himself citing an unnamed contemporary 8th-century Syriac source, Tiberius justified his coup by pointing to Leontius' own dethroning of Emperor Justinian II (r. 685–695, 705–711) for mismanaging the empire as precedent.

[19][20][21] Heraclius invaded the Umayyad Caliphate in late autumn of 698, crossing the passes of the Taurus Mountains into Cilicia before marching for northern Syria.

Heraclius defeated an Arab army sent from Antioch, then raided as far as Samosata before pulling back to the safety of Byzantine lands in spring of 699.

[25] In 705, Justinian led an army of Slavs and Bulgars to Constantinople and laid siege to it for three days before scouts discovered an old and disused conduit that ran under the walls of the city.

Justinian and a small detachment of soldiers used this route to gain access to the city, exited at the northern edge of the wall near the Palace of Blachernae, and quickly seized the building.

[29] Six months later, probably on 15 February,[21] Justinian had both Leontius and Tiberius dragged to the Hippodrome and publicly humiliated, before being taken away to the Kynegion (a city quarter near the Kynegos Gate) and beheaded.

[6] Head comments that although little is known of Tiberius, the evidence points to him being a "conscientious and effective ruler", and states that he might be remembered as "one of the truly great emperors of Byzantium" if he had reigned longer.

[33] The Byzantinists Cyril Mango and Roger Scott do not view this theory as likely, as it would mean that Emperor Theodosius had to have lived for thirty more years after his abdication.