Iorwerth Drwyndwn, known as Iorwerth mab Owain Gwynedd ("the flat-nosed", was the eldest legitimate son of Owain Gwynedd (the king of Gwynedd) and his first wife Gwladus ferch Llywarch. Owain had already other children born to various mistresses, but in c. 1128, a son, Iorwerth, was born to his wife. These parents' joy was abruptly dampened when they discovered their new son had a disfiguring birth defect. His entire nose was missing and his nostrils were mere holes in his face. For his entire life he would bear the nickname "Drwyndwn", meaning "broken nose". He received the same education and military training as his siblings, but due to his defect it proved hard to find him a wife.

Around 1160 Owain had discussed with king Madog ap Maredudd the idea of a marriage between Iorwerth and king Madog's daughter Marged , who also had some unknown physical defect. Madog would only accept the offer if Owain would confer a Lordship on Iorwerth to assure the solvency of the resulting new family unit. However, at the time Owain had no open lordships in his service and Madog died before any became open. But by 1168, the Lord of Nant Conwy, Nefydd Hardd, died leaving only teenage sons. Reopening talks with now king Gruffudd Maelor of Powys, it was agreed the long-proposed marriage between Iorwerth and Marged would proceed if Owain would grant the Nant Conwy lordship to Iorwerth, together with an appropriate manor in that commote. Iorwerth Drwyndwn and Marged ferch Madog were married, he 40 years old and she 26.

Iorwerth received Nant Conwy as his inheritance from his father, Owain Gwynedd, and was also permitted to retain his revenues as Lord of that commote. However, he did not receive the crown succession, as was the normal tradition, because of his nose defect.

After the death of Iorwerth's father, the throne was assumed by his eldest base son Hywel. The same month Hywel was killed by Dafydd, the eldest son of Owain's second wife. Dafydd not being of kingship age, the throne fell to Maelgwn. As Dafydd and his brother Rhodri reached kingship age, they forced Maelgwn into exile in Ireland. The following year Iowerth joined a group of Maelgwn supporters who sought to restore him to the throne.

Few ancient sources describe Iorwerth's death, though contemporary writers claim he died in battle at a time when his son was still a small child. John Edward Lloyd states that Iorwerth was killed in battle at Pennant Melangell, in Powys, in 1174 during the wars deciding the succession following the death of his father. Contrary to this, a contemporary elegy by the bard, Seisyll Bryffwrch states his grave was in Llandudclud, a church in Penmachno Nant Conwy. He says the effigy seen in Pennant Melangell is of a much later date than the 12th century and would appear to commemorate a 13th century descendat of Rhiryd Flaidd.

Iorwerth's widow choose to raise his only son Llewelyn in Gwynedd, where in a few years he could claim kingship by birthright. She appealed to other Gwynedd barons who had opposed the coup carried out by Dafydd and Rhodri Iorwerth's half brothers. His son would later become one of Wale's most famous monarchs, uniting the realm and dominating wales for 45 years. He became known as Llywelyn Fawr ("the Great").