Contemporary and near-contemporary accounts describe Momchil physically as "imposing in appearance", "as tall as two men" and, in the words of a Turkish poet, "resembling a
minaret". Persecuted by the Bulgarian authorities, some time before 1341 he deserted the Byzantine army and fled to Serbia to serve its ruler
Stephen Dušan. In Serbia, he formed a company of 2,000 trusted men, both Bulgarians and Serbs. Based on
Serbian epic poetry Momchil had a sister,
Alena, who later became
Queen Consort of Serbia, from 1365 until 1371. 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 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. At the time, Momchil was approached by agents of Kantakouzenos' opponents, the
Constantinopolitan regency, and persuaded to turn against him. Thinking that Kantakouzenos and his Turkish allies from the
Emirate of Aydin were far away in eastern
Thrace, he attacked a Turkish fleet of 15 ships near
Portolagos and sank three of them. He then overcame another Turkish force that arrived to exact retribution near the fortress of
Peritheorion (also known as Burugrad), and plundered several cities in the area that refused to surrender. Afterwards, Momchil along with 1,000 horsemen attacked Kantakouzenos, who had set camp near
Komotini with only 60 horsemen to protect him. The Byzantines were thoroughly routed: Kantakouzenos' horse was killed and he received a powerful hit to the head, which he survived thanks to his helmet. Momchil captured many of Kantakouzenos' men, but the claimant to the throne himself managed to escape in the turmoil. Soon, however, Momchil sent messages to Kantakouzenos asking for forgiveness. The latter, loath to alienate Momchil and open another front in his rear, pardoned him in exchange for promises of future good conduct, and even awarded him the title of
sebastokrator. Momchil nevertheless continued to entertain parallel ties with the regency, and even secured the title of
despotes from the Empress
Anna of Savoy. ==Ruler in the Rhodopes and death==