In the course of a bardic disputation early in Cynddelw's career (Myv. Arch., 154a), he was reminded that he had sprung from no family of poets, and although his opponent, Seisyll Bryffwrch (q.v.), was concerned in minimizing his achievements, he refers to him as ‘Cynddelw the big, giant of gatherings’; hence it is to be presumed that he was called ‘Mawr’ in the first place on account of his size. He had at least one son, named Dygynnelw, who was killed in battle (Myv. Arch., 185a).

Cynddelw was appointed leading poet to the court of Madog ap Maredudd, prince of Powys (d. 1160) (q.v.). His two best extant poems resulting from his connection with Madog's court are ‘In praise of Eve,’ who was the prince's daughter, and the fine series of eighteen englynion in which he laments the death of Madog and his son Llywelyn, with whom was buried the unity of Powys. Cynddelw then turned to sing the praises of Owain Gwynedd (q.v.), and his poetic gifts are seen at their height in the ‘Elegy to Owain,’ 1170. Henceforward, and until the end of the century, he sang to a number of the leading princes of Gwynedd, Powys, and Deheubarth, and in this respect he is the earliest poet known to have taken the whole of Wales as his field. He also sang awdlau to God, and his ‘Song to Tysilio’ is largely in praise of Meifod. His only other extant religious poem is the ‘Deathbed song.’ His poetry is mainly characterized by a quality described by W. J. Gruffydd as ‘primeval starkness.’

The last of his compositions which can be dated precisely is the series of englynion to Llywelyn ap Iorwerth (Myv. Arch., 189b), in which the rise of that prince (q.v.) is described as far as his capture of Mold in Jan. 1199. (His authorship of the later englynion entitled the ‘Circuit of Llywelyn’ is very dubious.) Part of his ‘Deathbed song’ is found in the ‘Black Book of Carmarthen,’ and a little of his work also appears in Pen. MS. 3, which also belongs to the first half of the 13th cent. Most of Cynddelw's extant work is found in the Hendregadredd manuscript, a considerable part in the ‘Red Book of Hergest,’ and all which has survived has been collected together in Myv. Arch.