During the Byzantine civil war of 1341-1347, Momchil joined the forces of John VI Kantakouzenos (r. 1341-1354), who had perhaps known Momchil during his flight to Serbia in 1342, at the beginning of the war. In 1343, as per the wishes of the local population, Kantakouzenos gave Momchil governance of the region of Merope in the Rhodope Mountains, a virtual no man's land plagued by nomadic Slavic brigands. In the words of Kantakouzenos himself, the appointment was because [Momchil] was of the same race these nomads would be favourably disposed toward him, but also because he was not lacking in courage and boldness in battle and was a first-rate expert in robbery and plunder. As the governor of Merope, Momchil gathered of an army of 300 cavalry and 5,000 infantry from different nationalities. Though he considered himself able to set against any side in the Byzantine war, along with Umur Bey's Turkish forces he nevertheless assisted Kantakouzenos in his campaigns in 1344.