Rhodri was the son of Merfyn Frych, who had claimed Gwynedd upon the extinction of Cunedda's male line. Rhodri then inherited the realm after his father's death around 844. Merfyn hailed from "Manaw" which may either refer to the Isle of Man or Manau, the ancestral homeland of all Gwynedd's kings since Cunedda.
According to later genealogies, his mother or grandmother was Nest ferch Cadell of the ruling dynasty in Powys. Although surviving texts of Welsh law expressly forbid inheritance along the maternal line, Nest and Rhodri's supposed inheritance was later used to justify Gwynedd's annexation of Powys after the c. 855 death of Cyngen ap Cadell in preference to Cyngen's other heirs.
Similarly, Rhodri's marriage to Angharad ferch Meurig was used to explain his supposed inheritance of her brother Gwgon's kingdom of Ceredigion after that king's death in 872 via a principle of jure uxoris that does not survive in our sources for Welsh law.
The Chronicle of the Princes records two victories by Rhodri in 872: the first at a place given variously as Bangolau, Bann Guolou, or Bannoleu, where he defeated the Vikings on Anglesey "in a hard battle" and the second at Manegid or Enegyd where the Vikings "were destroyed".
The Chronicle of the Princes records his death occurring at the Battle of Sunday on Anglesey in 873; the Annals of Wales record the two events in different years and Phillimore's reconstruction of its dates places Rhodri's death in 877. According to the Chronicle, Rhodri and his brother Gwriad were killed during a Saxon invasion (which probably would have been under Ceolwulf of Mercia, given that the Wessex forces under Alfred the Great were fighting Vikings in East Anglia at the time); after their death, the distraught women of the island grabbed their men's weapons and forced the Saxons to retreat. The Annals record no great details of the death, but where the B text calls Gwriad Rhodri's brother, the A text has him as Rhodri's son instead. It is likely he was killed in battle given that all the sources call his son Anarawd's victory over the Mercians at the Battle of the Conwy a few years later "God's vengeance for Rhodri".