He ruled until 698, when he was overthrown by Apsimarus, a droungarios who had taken part in a failed expedition that had been launched by Leontius to recover Carthage.

[4][5][6] Christian Settipani speculates that Leontius was the son of a certain Lazarus, who was the direct descendant of emperor Phocas and general Priscus.

[7] Leontius was appointed as strategos of the Anatolic Theme, at the time the most senior military command of the Byzantine Empire,[4][5][6][8] and patrikios by Emperor Constantine IV, possibly c. 682 AD.

[9][10][11][12][13] The civil war in the Umayyad Caliphate provided an opportunity for the Byzantine Empire to attack its weakened rival, and, in 686, Emperor Justinian II sent Leontius to invade Umayyad territory in Armenia and Iberia, where he campaigned successfully, before leading troops into Media and Caucasian Albania; during these campaigns he gathered loot.

[4][5] Leontius' successful campaigns compelled the Umayyad Caliph, Abd al-Malik ibn Marwan, to sue for peace in 688, agreeing to tender part of the taxes from Umayyad territory in Armenia, Iberia, and Cyprus, and to renew a treaty signed originally under Constantine IV, providing for a weekly tribute of 1,000 pieces of gold, one horse, and one slave.

[5][14][15][6] After further setbacks in the war, Justinian released Leontius in 695 because he feared losing control of Carthage, and appointed him strategos of the Theme of Hellas in Southern Greece.

[5][14] Leontius had wide support from the aristocracy, who opposed Justinian's land policies, which restricted the aristocracy's ability to acquire land from peasant freeholders,[18] and the peasantry, who opposed Justinian's tax policies,[5][18] as well as the Blue faction (one of the Hippodrome factions), and the Patriarch of Constantinople Callinicus.

[5][20] Apsimar took the regnal name Tiberius, gathered a fleet and allied himself with the Green faction, before sailing for Constantinople, which was enduring the bubonic plague.

[5][20] Leontius stayed in the monastery under guard until Justinian retook the throne with the assistance of the Bulgar king Tervel in 705.