Nothing is known of this man except what Guto says about him in the only poem addressed to him, poem 16. Sir was a title given in the Middle Ages to priests without a degree (it does not imply that William was a knight). The Trahaearn named in line 19 is in all probability Sir William’s father, but even so it is not possible to locate Sir William in the genealogies. Two men from Glamorgan called William ap Trahaearn are noted in WG2, but they were both married men with offspring, a fact difficult to reconcile with what Guto says about priestly chastity in this poem, and furthermore they belonged to generation 13, the same generation as William Herbert, second earl of Pembroke, who was not yet an adult in 1469. That makes them too late for this poem, which appears in manuscript c.1440 (see textual notes).
Guto suggests that Sir William is a native of the parish of Merthyr Tydfil itself (46) and that he has relatives there who are men of lineage (51–2). Since Sir William has spent a good sum restoring and beautifying the church, not to speak of welcoming poets to his house, it appears that he was quite well off. Guto also describes him as educated (cf. 19, 35–6, 70).