The following is adapted from material at a UK web-site, of the Cornwall County Council (which appears to bear no copyright symbol)
In the late Roman period Cornwall was known as Cornouia, the land of the Cornovii. Welsh sources point to a succession of Dumnonian Kings right through to the 9th century, and a 10th century memorial to King Ricatus stands in the grounds of Penlee House, Penzance. By this time, Cornouia has become Cornubia (Latin), Cernyw (Welsh) and Kernow (Cornish). The British language evolves in Dumnonia into what becomes Cornish.
In the 6th century, during the time of the invasions in the east of England by the "English" (Jutes, Angles, Saxons, Hengist, Horsa and other Continental hordes) (the period of Vortigern, Uther Pendragon, Arthur, and the long retreat westwards), Cornwall was going through the "age of Saints", when Christian missionaries appear, in their ones and ones, to have entered Cornwall to preach the gospel and found tiny chapels and baptistries. Thus, around 600CE St.Piran's Oratory was founded, on Penhale Sands, and was later to be buried under those same sands, thereby preserving it until the 19th Century.
In or around 577CE a battle ("Doerham Down") near Bristol, resulting in a Saxon victory, led to the separation off of the Welsh from the Cornish or West Welsh.
A century later, with the arrival of the "English" at the Bristol Channel the separation became physical and enforced. By 710CE the city of Exeter (formally a Roman legionary centre) had fallen to the Saxons and thereafter Ina of the West Saxons led half a century of incursions westwards, seeking to wipe out the native kingdoms.
In the following century the Cornish, in concert with Danish Viking forces skimished with the Saxon armies, but were unsuccessful in achieving more than a delay in their advance.
However when, in 927CE, Athelstan re-occupied Exeter there is no record of him forcing his way further west, and it may be that, by paying tribute to him, the Cornishmen obtained a high degreee of autonomy. In 931 he was to institute a bishopric at St.Germans, although in 1042 the bishopric was united with that of Crediton and removed to Exeter and, ecclesiastically, for the rest of Period, the county was no more than an archdeaconry.
In 936 Athelstan settled the river Tamar as the boundary between Saxon Wessex and Celtic Cornwall, and in 1066, after the Conquest, Cornwall formally became a county, under Robert of Mortain as its Earl.
In the early part of Medieval history Cornwall enters the historical record mostly in terms of grants of charters for towns, fairs, markets and occasional famines or the evolution of the tin-mining industry.
in 1497 Cornishmen revolted against Henry VII's taxes and set off to march on London but were routed en route. Later that year the pretender Perkin Warbeck landed at Whitesand Bay, near Land's End, seeking to make good his claim to be Richard IV.
In 1512 died a Cornishwoman, Thomasine Bonaventure (later Percival, and a Dame) who had been Lord Mayoress of London.