His father, Gruffudd ap Cynan, was a strong and long-lived ruler who had made the principality of Gwynedd the most influential in Wales during the sixty-two years of his reign, using the island of Anglesey as his power base.

Owain and Cadwaladr, in alliance with Gruffydd ap Rhys of Deheubarth, won a major victory over the Normans at Crug Mawr near Cardigan in 1136[5] and annexed Ceredigion to their father's realm.

On behalf of his father, Gruffudd ap Cynan, Gwynedd directed military operations to the "cantrefs of Meirionnydd, Rhos, Rhufoniog and Dyffryn Clwyd to Gwynedd proper", and it was against the Normans, with Gruffydd ap Rhys he secured a victory at the Battle of Crug Mawr and the temporary occupation of Kingdom of Ceredigion.

Owain took advantage of The Anarchy, a civil war between Stephen, King of England, and the Empress Matilda, to push Gwynedd's boundaries further east than ever before.

The prince of Powys, Madog ap Maredudd, with assistance from Ranulf de Gernon, 4th Earl of Chester, gave battle at Coleshill, but Owain was victorious.

Owain's men ambushed the royal army in a narrow, wooded valley, routing it completely with King Henry himself narrowly avoiding capture.

Forty years after these events, the scholar Gerald of Wales, in a rare quote from these times, wrote what Owain Gwynedd said to his troops on the eve of battle: "My opinion, indeed, by no means agrees with yours, for we ought to rejoice at this conduct of our adversary; for, unless supported by divine assistance, we are far inferior to the English; and they, by their behaviour, have made God their enemy, who is able most powerfully to avenge both himself and us.

However, apart from a small melee at the Battle of Crogen, there was little fighting, for the Welsh weather came to Owain's assistance as torrential rain forced Henry to retreat in disorder.

[11] Henry did not invade Gwynedd again, and Owain was able to regain his eastern conquests, recapturing Rhuddlan Castle in 1167 after a siege of three months.

Rhun was Owain's favourite son,[4] and his premature death in 1146 plunged his father into a deep melancholy, from which he was only roused by the news that his forces had captured Mold Castle.

According to legend, one of Owain's sons was Prince Madoc of Wales (Madog), who is popularly supposed to have fled across the Atlantic and colonised America; this tradition is part of the Pre-Columbian transoceanic contact theories.

Owain appears as a minor character in novels of Sharon Kay Penman concerning Henry II and Eleanor of Aquitaine (When Christ and His Saints Slept and Time and Chance).