The continued success of Ivaylo against Byzantine reinforcements led Ivan Asen III to flee the capital and escape to the Byzantine Empire, while George Terter I seized power as emperor in 1280. With the threat from Ivaylo and Ivan Asen III removed, George Terter I made an alliance with King Charles I of Sicily, with Stefan Dragutin of Serbia, and with Thessaly against Michael VIII Palaeologus of the Byzantine Empire in 1281. The alliance failed as Charles was distracted by the Sicilian Vespers and the secession of Sicily in 1282, while Bulgaria was ravaged by the Mongols of the Golden Horde under Nogai Khan. Seeking Serbian support, George Terter I engaged his daughter Anna to the Serbian king Stefan Uroš II Milutin in 1284.
Since the death of the Byzantine Emperor Michael VIII Palaiologos in 1282, George Terter I re-opened negotiations with the Byzantine Empire and sought the return of his first wife. This was eventually accomplished by treaty, and the two Marias exchanged places as empress and hostage. Theodore Svetoslav also returned to Bulgaria after a successful mission of Patriarch Joachim III and was made co-emperor by his father, but after another Mongol invasion in 1285, he was sent off as a hostage to Nogai Khan. Theodore Svetoslav's other sister, Helena, was also sent to the Horde, where she married Nogai's son Chaka.
The reasons for his exile are not very clear. According to George Pachymeres, after an attack by Nogai Khan on Bulgaria, George Terter was removed from the throne and then traveled to Adrianople. The Byzantine Emperor Andronikos II Palaiologos at first refused to receive him, perhaps fearing complications with the Mongols, and George Terter was kept waiting in wretched conditions in the vicinity of Adrianople. The former Bulgarian emperor was eventually sent to live in Anatolia. George Terter I passed the next decade of his life in obscurity.