The eldest son of King Urien of North Rheged, Owein was a giant of a man, best known for his association with High-King Arthur, at whose court he is said to have spent much of his youth. This, however, was after initial rivalry between the two. Peace only followed a fierce battle between them described in the Dream of Rhonabwy. Here Owein and Arthur played Gwyddbwyll, an ancient form of chess, while their armies fought on the field of battle. Owein's Ravens, as his mounted followers were called, were nearly defeated until their King raised his flag and encouraged them to fight on with renewed vigour. After Arthur's defeat, Owein agreed to lead the High-King's own warband. Owein's ravens are remembered in his coat of arms, still used by some of his supposed descendants today.

Owein is the hero of one of Chrétien's best-known works, Yvain, which was based on the old Welsh poem, Iarlles y Ffynnawn: While out looking for adventure in Brittany, Owein discovered a magical fountain in the Forest of Broceliande. Here he encountered and fought with a warrior named Esglad, who was eventually chased back to his stronghold, not far away. Esglad died of his wounds, and Owein married his beautiful widow, Laudine. The two were happy together for a while, but Owein soon felt compelled to return to his High-King and fight against the enemies that threatened Britain's independence. Unfortunately, by the time he returned, Laudine rejected him and he went mad with remorse.

When his sanity returned, Owein crossed the channel to resume his role at his father's side defending the borders of their Kingdom, notably at the Battle of Argoed Llwyfain (Leeming Lane, Yorks) where he killed the great King Theodoric Fflamddwyn (the Firebrand) of Bernicia. He was now accompanied everywhere by a lion he had supposedly helped to defeat a huge serpent. At Caer-Ligualid (Carlisle), he married again, to Penarwen ferch Culfanawyd, but the lady was unfaithful to him and he looked elsewhere for comfort. He fell for the young Princess Thaney, daughter of King Lot Luwddoc (of the Host) of Gododdin, but, as a married man, their love was forbidden. When it was found that poor Thaney was pregnant she underwent extraordinary tortures before the two were finally able to unite in safety, in Rheged.

After his father's assassination, in 590, Owein only managed to hang onto his Kingdom for a few years. He was under intense pressure from Urien's old British enemies-turned-allies-turned-enemies. His brother, Elffin, was attacked by King Gwallawc Marchawc Trin (the Battle Horseman) of Elmet; Owein himself with his brother, Pasgen, had to fight off King Dunaut Bwr (the Stout) of the North Pennines. Then King Morcant Bulc of Bryneich and, Bran, possibly his brother, moved in for the kill. Owein fell and so did Rheged.

Owein was buried in either Llan-Forfael or Lan-Heledd, neither of which have been identified, but tradition would indicate the Churchyard of St. Andrew in Penrith, where his supposed grave is still pointed out. Rheged thus fell into chaos and the Kingdom contracted considerably. Notable was the loss of Catreath to the Northumbrians. If Owein's brothers or sons ever managed to rule as Kings, details of their reigns are lost.