Clydno was a prince of Strathclyde, the heir of Cunbelin, a younger son of King Dumnagual Hen. The alternative recording of his father as Cynwyd Cynwydion an be dismissed as an error. With ancient Celtic inheritance laws calling for the division of estates amongst sons, it seems that Strathclyde may have been rather short of land. Cunbelin or Clydno must have taken up arms against the adjoining kingdom of Gododdin in order to establish a domain for themselves around what thence became known as Din-Eitin (Edinburgh).
During the AD 560s, Clydno joined the grandsons of his uncle, King Cedic of Strathclyde, in raiding Gwynedd to exact revenge for the killing of another cousin, Elidyr Mwynfawr. They devastated the country around Caer-Segeint (Caernarfon) but were eventually driven out by King Rhun Hir.
Two daughters, Eurneid and Euronwy, are mentioned in various records; but most famous was his son, Cynan, who fought at Catraeth with Clydno's successor, Mynyddog Mwynfawr, presumably an older relative.