Glywysing is said to be named after Glywys, a real or legendary early monarch, whose name may continue that of the Romano-British *Glevenses, the territory and citizens of Glevum (modern Gloucester). According to 12th-century sources, after the death of Glywys, the kingdom was divided into seven cantrefs named for his sons:Cydweli, Gwyr, Margam, Penychen, Gwynllwg, Gorfynydd, and another. These were typically ruled together by the head of the family and sometimes treated as appenage subkingdoms.
The borders changed over time, but it is generally thought that its lands originally lay between the Afon Llwyd and the River Towy. At times they expanded eastwards to encompass both Gwent and Ergyng, but some time before the early 8th century, Cydweli and Gwyr (Gower) were lost to Dyfed. Today the area of Glywysing is known as Glamorgan.
In the mid 10th century, the kingdom merged with Gwent and changed its name to Morgannwg or Gwlad Morgan in honour of King Morgan the Old (r. 942–74) or his ancestor King Morgan the Generous (fl. c. 730). Glywysing seems to have been a sub-kingdom or principality of the Kingdom of Morgannwg, along with Gwent. After the death of Morgan the Old, Gwent and Glywysing were separated again from 974 to 1055, but Glywysing alone was often referred to as Morgannwg. Both areas were conquered by Gruffydd ap Llywelyn in about 1055, but on his death in 1063, Morgannwg, the union between Gwent and Glywysing, was reconstituted. How this occurred is unclear; possibly the Kings of Glywysing were also Kings of Morgannwg and the Kings of Gwent were semi-independent under-Kings. The last native ruler of these areas was Iestyn ap Gwrgan, King of Morgannwg (1081-1090), who was deposed by Robert Fitzhamon.