He married, firstly, Gwladys, the daughter of Llywarch ap Trahaearn; and secondly, Christina, his cousin, the daughter of Goronwy ap Owain ‘the Traitor,’ Lord of Tegeingle, to whom he remained constant despite the active disapproval of the Church. He had eight sons (Iowerth, Rhun, Cynan, Iefan, Maelgwn, Madog, Rotpert & Idwal) and two daughters (Gwenllian wife of Owain Cyfeiliog and another) by the first lady; and two sons (Dafydd and Rhodri) and one daughter (Angharad wife of Gruffudd Maelor I) by the latter. He also had another relationship with Pyfog of Ireland, by whom he had a son, Hywel.

As a young man in the 1120s, Owain was largely associated with his elder brother, Cadwallon, in restoring Gwynedd’s prosperity on behalf of their ageing father. Together, they directed the military campaigns which added Meirionydd, Rhos, Rhufoniog and Dyffryn Clwyd to Gwynedd proper. Thus, at his accession to the throne, upon Gruffudd's death in 1137 - Cadwallon having died five years earlier - the groundwork for an impressive career had already been firmly set.

Political anarchy in England had already provided Owain the opportunity to combine his forces with those of Gruffudd ap Rhys and others. Together, they inflicted defeat on the Normans at Crug Mawr, in 1136, and temporarily occupied Ceredigion. Owain's campaigns in South Wales, however, were largely intended as a diversionary tactic designed to obscure his main objective of territorial consolidation in the North. Eventually, despite the opposition of Earl Ranulf of Chester and Prince Madog ap Maredydd of Powys, the area surrounding Mold submitted to him in 1146 and, three years later, Tegeingl and Ial followed suit. By 1157, however, the situation over the border England had changed considerably and Owain suffered his one and only decisive reversal at the hands of King Henry II of England.

Though the English King’s expedition to Gwynedd was militarily indecisive, it marked a new and positive stage in relations between the two Kingdoms. Deprived of Tegeingl and Ial, Owain was forced to accept the return of his exiled brother, Cadwaladr, and offer him a share of power. However, with characteristic prudence and insight, Owain realised the great potential of a friendly relationship with the Plantagenet monarchy. He did homage to King Henry and seems to have agreed to change his status from 'King' to mere 'Prince'. Owain, further, made no attempt to break his new-found feudal link with the English when, at the climax of his reign after the general Welsh uprising of 1165, he destroyed the royal strongholds of Tegeingl and re-established the power of Gwynedd along the Dee estuary. For Owain regarded himself as no ordinary vassal (as shown by his attitude to Episcopal elections in Bangor). He gave clear direction to the policies of his successors, enabling the Welsh Kings to take their place alongside the great feudal magnates of the time.

Owain Gwynedd died on 28th November 1170, and was buried in Bangor Cathedral where his traditional tomb may still be seen.