Through his opportunistic involvement in the Byzantine civil war of 1341–1347, where he played the various sides against each other, he became ruler of a large area in the Rhodopes and western Thrace.

Momchil achieved initial successes against Turks and Byzantines alike, setting Turkish ships on fire and almost managing to kill one of his main opponents at the time, John VI Kantakouzenos.

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".

[1] According to a contemporary source, Momchil was a native of the "border area of Bulgarians and Serbs", which at the time straddled the Rhodopes and the Pirin mountains.

This was a main factor in his decision to join a band of brigands (hajduks) which was active in the scarcely governed border areas between Bulgaria, Byzantium and Serbia.

[14] 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.

[12][13][15] 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.

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.

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.

However, to the surprise of Kantakouzenos and Umur Bey, the majority of Momchil's men were standing in front of the city walls and had not yet participated in the skirmishes.

[10] In Bulgarian and South Slavic folklore in general, Momchil is glorified in numerous songs and epic tales as a brigand, defender of the people and a prominent fighter against the Turks.

[24] In some folklore material, Momchil, referred to as a duke, acts as the uncle of Prince Marko, another legendary figure who in epic poetry is a fighter against the Turks.