The most famous of the Kings of the North, and possibly one of the earliest Christian Kings. One of the sons of Cynfarch Oer (the Dismal), Urien appears to have united a Kingdom that was originally either divided between, or shared with his brothers, Llew and Arawn. These latter appear to have held sway north of the Solway Firth, possibly moving into King Gwendoleu's old Kingdom and also conquering King Senyllt's Galloway, where still stands Din-Rheged (Dunragit). Urien's power-base was at Caer-Ligualid (Carlisle), though he also had a palace at Llwyfenydd on the River Lyvennet, and probably at Caer-Brogwm (Brougham) and Pen Rhionydd (possibly near Stranraer) too. The heart of his kingdom was modern Cumbria, which even today is named after the British Cymri, though his Kingdom, at one time, appears to have stretched as far north as Murief (Moray). Tradition asserts that his court played host to the High-King Arthur whenever he was travelling through the North, and Urien is thought to have married Arthur's half-sister, the enigmatic Morgan Le Fay. The King of Rheged, however, must have been something of a toy-boy, even if Morgan was the High-King's neice as some sources insist. A geneaological based birth date of AD 490 for the king is historically unacceptable. It is possible, even probable, that Urien's wife was a different lady, Modron ferch Afallach, though this name has unfortunate immortal overtones.

There is some controversy as to whether or not Urien Rheged should be identified with King Urien of Gore. Goodrich thinks that Gore was an area of Ynys Manaw (Isle of Man) that the King of North Rheged conquered though her reasoning is flawed. Bruce makes a good case for it being the whole island of Man based on Chretien's geography and description. The name may derive from the French or Welsh words for Glass, indicating an underworld connection. The identification with Gwyr (Gower) in South Wales is probably derived from the fact that Urien's son, Pasgen, later settled there. Urien was a great patron of the arts, particularly the works of his personal bard, the famous Taliesin.

Throughout his time as King, Urien's relations with his fellow British monarchs were erratic. He made many raids on rival kingdoms as far north as Manau Gododdin and once captured King Selyf Sarffgadau (Battle-Serpent) of Powys in battle. However, towards the end of a long reign, Urien led a coalition of British Kings against the expanding Saxons. His allies included Kings Riderch Hael (the Generous) of Strathclyde, Gwallawc Marchawc Trin (the Battle Horseman) of Elmet and, probably, Morcant Bulc of Bryneich. Many battles were fought including Gwen Ystrad and the Cells of Berwyn. This latter, probably fought at the Roman Fort of Brememium (High Rochester) may have later been turned into High-King Arthur's supposed 11th Battle, of Breguoin. After the defeat of the Yorkist Kings, Peredyr Arueu Dur (Steel Arms) and Gwrgi, by the Bernicians in 580, Urien was quick to claim the strategic region around Catraeth, before the Saxons of Bernicia and Deira were able to secure the area and unite their two peoples. This struggle may have culminated in the Battle of Argoed Llwyfein (Leeming Lane, Yorks). It was at this battle that King Theodoric Flamddwyn (the Firebrand) of Bernicia was killed by Urien's son. The British probably held both the old Roman fort of Catraeth itself and the hillfort site at Richmond.