After the murder of Leo and usurpation of the throne by Michael the Amorian, Thomas revolted, claiming the throne for himself. Thomas quickly secured support from most of the themes (provinces) and troops in Asia Minor, defeated Michael's initial counter-attack and concluded an alliance with the Abbasid Caliphate. After winning over the maritime themes and their ships as well, he crossed with his army to Europe and laid siege to Constantinople. The imperial capital withstood Thomas's attacks by land and sea, while Michael II called for help from the Bulgarian ruler khan Omurtag. Omurtag attacked Thomas's army, but although repelled, the Bulgarians inflicted heavy casualties on Thomas's men, who broke and fled when Michael took to the field a few months later. Thomas and his supporters sought refuge in Arcadiopolis, where he was soon blockaded by Michael's troops. In the end, Thomas's supporters surrendered him in exchange for a pardon, and he was executed.

Thomas's rebellion was one of the largest in the Byzantine Empire's history, but its precise circumstances are unclear due to competing historical narratives, which have come to include claims fabricated by Michael to blacken his opponent's name.