Geoffrey of Monmouth was born sometime around 1100, probably in Monmouth in southeast Wales, though his family was undoubtedly from France, having come with the Norman conquest of Britain. “His father was named Arthur. Geoffrey was appointed archdeacon of Llandsaff in 1140 and was consecrated bishop of St. Asaph in 1152. He died c. 1155.

Geoffrey is one of the most significant authors in the development of the Arthurian legends. It was Geoffrey who, in his Historia Regum Britanniae (completed in 1138) located Arthur in the line of British kings. Such an action not only asserted the historicity of Arthur but also gave him an authoritative history which included many events familiar from later romance. Geoffrey also introduced the character of Merlin as we know him into the legends. Geoffrey’s Merlin, a combination of the young and prophetic Ambrosius in Nennius’s history and the prophet Myrddin who figures in several Welsh poems, first appears in a book known as the Prophetiae Merlini (The Prophecies of Merlin), which was written about 1135 but then incorporated as Book VII of the Historia. This book contains the prophecies made by Merlin to Vortigern, which foreshadow not only the downfall of Vortigern but also the rise and fall of Arthur, events subsequent to the end of the Historia, and events of the obscure future.”  http://www.lib.rochester.edu/camelot/geoffrey.htm

“Modifying the name of the northern bard Myrddin, Geoffrey uses Welsh predictions of a Celtic revival and many of his own probable invention and ascribes them to the prophet. This work was followed toward 1136-1138 by the Historia Regum Britanniae that incorporated the prophecies in it. Near the end of 1150 he composed a long narrative poem expanding on Welsh traditions about the prophet entitled, Vita Merlini (“Life of Merlin”).”  http://www.pantheon.org/articles/g/geoffrey_of_monmouth.html

By his late twenties, Geoffrey certainly seems to have travelled eastwards to work at the Collegiate Church of St. George at the castle in Oxford.   He remained there, as a tutor of some kind, for at least the next twenty years  and began writing not long after he arrived.  The Prophecies of Merlin appear to have been a series of ancient Celtic prophecies which, at the request of Alexander of Salisbury, Bishop of Lincoln, Geoffrey translated into Latin, perhaps with some additions of his own. “Whether they had previously been attributed to the Northern British bard, Myrddin, is unknown. As with all his works, Geoffrey hoped the prophecies might bring him a lucrative preferment in the Church, and he used its dedication to ingratiate himself with Alexander who was Bishop of his local diocese. Geoffrey made a more appreciative acquaintance while at St. George’s, in the person of Walter the Provost, who was also Archdeacon of the city. In his writings, Geoffrey tells us that Walter gave him “a certain very ancient book written in the British language” and, probably because he was unable to read Welsh (or Breton) himself, the Archdeacon encouraged Geoffrey to translate it into Latin.”

Geoffrey began writing History of the Kings of Britain’ dedicated to Robert, Earl of Gloucester, and Waleran, Count of Mellent in 1136. “At the time, the work was taken at face value and accepted by most as a true history of the Welsh nation from around 1100 bc to around AD 689. Merlin appeared again, as an advisor to Kings Ambrosius and Uther, but the work was most notable for its extensive chapters covering the reign of the great King Arthur. Since the 17th century, however, its author has been largely vilified as an inexorable forger who made up his stories “from an inordinate love of lying”. Modern historians tend to be slightly more sympathetic.

At the end of 1150, Geoffrey appears to have come into the possession of further source documents concerning the life-story of his original subject, the bard, Myrddin (alias Merlin). Unfortunately, these did not line up terribly well the information he had given about this man in his History of the Kings of Britain – perhaps indicating that this part was either invented or, more probably, that Merlin’s name had been rather over-eagerly attributed to an otherwise unknown Royal adviser. Keen to put across the true story, without losing face, Geoffrey wrote the Life of Merlin, correctly placing its events after the reign of Arthur, but thus giving his title role an impossibly long lifespan. It was dedicated to his former colleague at St. George’s, Robert De Chesney, the new Bishop of Lincoln.

“Whenever his dates are checked, as in the Roman period, Geoffrey emerges clearly as a writer of fiction and cannot be relied upon for facts. Following medieval tradition, he fully modernizes Arthur’s court to the 12th century. Later, however, from Caesar on he is using what passed for real history at the time and some of his source materials can be identified – the Historia Brittonum, Bede and Gildas in addition to Roman historians.

For the most part he is creating and aggrandizing very little data but in his preface he claims to be translating from a much fuller source, one “ancient book in the British language” (maybe Welsh but probably Breton) bestowed upon him by Walter, archdeacon at Oxford. This claim remains dubious as no copy of this source is extant. But the tale of Arthur scribed by Geoffrey cannot be fully accounted for from the aforementioned sources hinting at some unknown text of some kind. There is a possible tie to the Continent from the resonance with 5th century events in Gaul. Traces of a similar source are found in the preface to the Breton Legend of St. Goeznovius.”  http://www.pantheon.org/articles/g/geoffrey_of_monmouth.html