[7] He may have come from the same French-speaking elite of the Welsh border country as Gerald of Wales, Walter Map, and Robert, Earl of Gloucester, to whom Geoffrey dedicated versions of his History.

[8] Sir Frank Stenton and others have suggested that Geoffrey's parents may have been among the many Bretons who took part in William the Conqueror's conquest and settled in the southeast of Wales.

[8] Geoffrey's structuring and shaping of the Merlin and Arthur myths engendered their vast popularity which continues today, and he is generally viewed by scholars as the major establisher of the Arthurian canon.

[14] Much of it is based on the Historia Britonum, a 9th-century Welsh-Latin historical compilation, Bede's Ecclesiastical History of the English People, and Gildas's 6th-century polemic De Excidio et Conquestu Britanniae, expanded with material from bardic oral tradition and genealogical tracts, and embellished by Geoffrey's own imagination.

For example, Giraldus Cambrensis recounts the experience of a man possessed by demons: "If the evil spirits oppressed him too much, the Gospel of St John was placed on his bosom, when, like birds, they immediately vanished; but when the book was removed, and the History of the Britons by 'Geoffrey Arthur' [as Geoffrey named himself] was substituted in its place, they instantly reappeared in greater numbers, and remained a longer time than usual on his body and on the book.

"[18] Geoffrey's major work was nevertheless widely disseminated throughout medieval Western Europe; Acton Griscom listed 186 extant manuscripts in 1929, and others have been identified since.

[21] The main conclusion of the study was that the Historia Regum Britanniae appears to contain significant demonstrable archaeological fact, despite being compiled many centuries after the period that it describes.

Geoffrey seems to have brought together a disparate mass of source material, including folklore, chronicles, king-lists, dynastic tables, oral tales, and bardic praise poems, some of which was irrevocably garbled or corrupted.

[22] Geoffrey's earliest writing was probably the Prophetiae Merlini (Prophecies of Merlin) which he wrote before 1135, and which appears both independently and incorporated into The History of the Kings of Britain.

The story is set long after the timeframe of the History's Merlin, but the author tries to synchronise the works with references to the mad prophet's previous dealings with Vortigern and Arthur.