Geoffrey of Monmouth a Welsh cleric gives the earliest story of Arthur's life in Historia Regum Britanniae (History of the Kings of Britain) in 1135 AD. Although Geoffrey of Monmouth is not the earliest source that mentions Arthur, it is the first to identify him as a high king from Britain's past.
Geoffrey of Monmouth claimed that he was merely copying an ancieint manuscript he had in his possession, but he appears to have invented most of his story, although elements are beleived to have existed in Celtic myth. The "certain very ancient book written in the British language." referred to by Geoffrey of Monmouth has never been found, and its very existence has been called into question.
In his History, Geoffrey of Monmouth details Arthur's genealogy, conception, birth, childhood, ascension to the throne, military conquests, and death. If Geoffrey of Monmouth is talking about a real historical figure, it would place Arthur's life as being from a birth in the late fifth century to a death in 542 AD, when the king was mortally wounded in his last battle.
Geoffrey's stories of Arthur then at became the basis of the Arthurian legend that was later elaborated by Chrétien de Troyes and Thomas Malory.
He has also used earlier sources such as Gildas, Nennius, The Annales Cambriae and Bede. None of these works mention such wealth of detail on Arthur as Geoffrey of Monmouth does. Only Gildas, Celtic monk-historian who died in AD 570, is anything like contemporaneous with Arthur, and he does not mention the name "Arthur".
The general conclusion is that the work of Geoffrey of Monmouth is romance rather than history.
King Arthur in Literature