He was the son of Rhys and brother to Anarawd, Cadell, Maredudd. After the death of their father in 1137 all four brothers ruled and worked to expand the kingdom and raid neighbouring princes. Following the death (Anarawd (1143) and Maredudd (1155)) or disablement (Cadell) of all his brothers he became sole ruler of the kingdom by 1155.
The failure of the English troops in Wales and troubles in England helped Rhys to prosper. After the death of Owain Gwynedd in 1170 he was acknowledged leader of the Welsh princes. In 1171 he signed a pact with Henry II and helped the king suppress the rebellion of 1173-74. After Henry's death Rhys was again active, revolting against Richard I and deeply involved in internal Welsh feuds. He was succeeded by Gruffydd ap Rhys.