A distant cousin of the previous king, Rhodri Molwynog, both claimed descent from Cunedda, the semi-legendary progenitor of the royal line of Gwynedd. In the case of Caradog, that descent ran through Cunedda's grandson (and son of Einion Yrth, the second king of Gwynedd), Owain Danwyn, the ruler of Rhos, a small cantref within the kingdom of Gwynedd. Caradog retained his line's status as provincial overlords of Rhôs, and it is from this position that he was eventually able, upon Rhodri's death, to claim the throne of the entire kingdom.
The exact date for Caradog's usurpation is uncertain, but it must have been between the year of Rhodri's death in 754 and 768. It was in that year that Elfoddw, the bishop of Gwynedd, convinced the Welsh church to adopt the Catholic formula for calculation the date of Easter, which had been the locus of a century-long rift between the Roman and Celtic churches since the Synod of Whitby. Although Caradog's name is not explicitly mentioned in conjunction with this event, Elfoddw's successful persuasion of the Welsh clergy strongly suggests the influence of a powerful secular leader, and Caradog is basically the only Welsh leader from this period who could have conceivably filled that role. Furthermore, it seems to suggest that Caradog was reasonably secure in his role by that point, which suggests (if somewhat inconclusively) that he had already been in power for some time.
As was common for Welsh kingdoms during this period, Gwynedd under Caradog was under constant threat by neighboring Anglo-Saxon kingdoms in England. The most commonly accepted explanation of Caradog's death has him falling to Coenwulf, king of Mercia, during battle in Snowdonia. Other records, however, suggest that he was murdered by strangulation, perhaps at the instigation of Cynan Dindaethwy, the son of Rhodri, who subsequently assumed control of Gwynedd. In some genealogies Caradog is identified as the father of Hywel Farf-Fehinog, who would, in turn, succeed Cynan to the throne. Other sources however identify Hywel as another son of Rhodri Molwynog.