After his father's death in 1322, he became actively involved in the civil war in the Byzantine Empire, in which the throne was being contested between Andronikos II Palaiologos and his grandson Andronikos III Palaiologos.

Taking advantage of the situation, George Terter II invaded Byzantine Thrace and, encountering little, if any, resistance, conquered the major city of Philippopolis (Plovdiv) and part of the surrounding area in 1322.

A Bulgarian garrison was installed under the command of a general named Ivan the Russian, and a court scribe praised George Terter II as a "possessor of the Bulgarian and the Greek sceptre".

A new campaign later the same year brought the subjugation of several fortresses around Adrianople, but the Bulgarians were now turned back and defeated by Andronikos III.

The Byzantine emperor was preparing for an invasion of Bulgaria, when he heard the news that George Terter II had died, apparently of natural causes.