Hywel Greasy-Beard was probably still quite young when his father, King Caradog ap Meirion of Gwynedd, died in AD 798. Unfortunately, Caradog had usurped the throne from the rightful Royal dynasty forty-four years before, so it probably came as no surprise when their heir, Cynan Dindaethwy, reclaimed his crown. With the help of his supporters, Hywel may have been able to retain control of his family's power-base in Rhos, but he was obliged to keep himself to himself until he was old enough to make his move.

In AD 813, Hywel raised an army and marched on Ynys Mon (Anglesey). He was defeated by Cynan, but did not give up. He returned the following year, drove the King of Gwynedd from his island shores and proclaimed himself King. Cynan tried to retake his throne in AD 816, but was killed in battle. Hywel ruled peacefully for the next nine years. He left no obvious heir at the time of his death, in AD 825, and the throne was seized by Cynan's maternal grandson, King Merfyn Frych of Ynys Manaw (Isle of Man).