Gwynllyw was the son of King Glywys, whose powerful kingdom of Glywysing was centred on Glamorgan, and supposedly extended as far east as the River Towy. He was a descendant of Macsen Wledig according to some accounts, while his mother Guaul was equally distinguished, being the granddaughter of Cunedda. The kingdom was split on Glywys' death amongst his sons, of whom Gwynllyw was the eldest and most powerful, and he was overlord over the others. The central area of his rule consisted of the cantref of Gwynllwg that was named after him and later known in English as Wentloog hundred. One of Gwynllyw's brothers was Saint Petroc, an important Cornish, and Breton saint and patron saint of Devon.