Rhys was the youngest son of Gruffydd ap Rhys, a prince of Deheubarth, and of Gwenllian, a sister of Owain Gwynedd. He was probably born in Ireland. Both his parents died when he was around four years old, Gwenllian as a result of leading her husband's army into battle in his absence, Gruffydd himself on his return. Following the deaths of his elder brothers, Rhys inherited the land they had won back from the English in 1155, and proceeded to challenge the new king of England, Henry II (reigned 1154 - 1189). Having successfully held out against Henry for a year or two, Rhys was defeated and most of his territory was taken from him. This was the point at which he agreed to pay homage to Henry, and peace temporarily reigned.
In 1162, Rhys took advantage of Henry's absence in Normandy to attempt to recover some of his lost lands. This resulted once again in defeat, following which Rhys was taken to England as a prisoner. Once released, however, he went back to his old defiant ways. Henry, preoccupied with domestic problems, could not respond effectively, and Rhys, now calling himself "Prince of South Wales", eventually agreed to negotiate in 1171, and an alliance ensued. Following Henry's death in 1189, Rhys found it difficult to maintain his position, and was obliged to enter into a protracted struggle against the Norman lord, William de Braose. He died peacefully, an old man at the peak of his power, in 1197.