Local tradition makes Geoffrey a Benedictine monk at Monmouth Priory, if not the actual prior. However, this seems to be due to a misidentification with his contemporary, Prior Geoffrey the Short of Monmouth. Certainly 'Geoffrey's Window' at which he is said to have sat and written his famous works and 'Geoffrey's Study' used as a schoolroom within the Priory Gatehouse are only of late 15th century date. At most it seems that Geoffrey might perhaps have been educated at Monmouth Priory. Some say, erroneously, that his tutor was an uncle named Uchtryd who made him Archdeacon of Llandeilo or Llandaff when he became Bishop of the latter in around 1140.

A variety of obscure medieval records give only glimpses of the man's real life. By his late twenties, Geoffrey certainly seems to have travelled eastwards to become a secular Austin canon at the Collegiate Church of St. George at the castle in Oxford. He was a member of the college community there, and a tutor of some kind, for at least the next twenty years - witnessing a number of charters during his residence - but he turned to writing not long after his arrival. 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.

So, in about 1136, the Welshman set about writing his 'History of the Kings of Britain' dedicated to Robert, Earl of Gloucester, and Waleran, Count of Mellent. Whether this was a straight translation of an 'ancient book' or contained considerable embellishments, if not worse, from Geoffrey himself has been the subject of heated debate for many generations. 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. Parts of Geoffrey's work certainly seem to have their origins in ancient Celtic mythology, others could have come from works by authors such as Gildas, Nennius, Bede and also the Mabinogion. But there are also hints that he had access to at least one other work unknown to us today. His 'King Tenvantius of Britain,' for example, was otherwise unknown to historians until archaeologists began to uncover Iron Age coins struck for a tribal leader in Hertfordshire named Tasciovantus. Some people consider the several copies of a Welsh version of Geoffrey known as the 'Brut y Brenhinedd' to be his original 'ancient book'. However, the 'Chronicle of Saint Brieuc' makes reference to several of Geoffrey's characters apparently from a source called the 'Ystoria Britannica'.

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.

The following year, Geoffrey's sycophancy at last paid off. He was elected Bishop of St. Asaphs, for good service to his Norman masters; and was consecrated by Archbishop Theobald at Lambeth Palace in February 1152. As a Welsh-speaker, he was probably chosen in an attempt to make the diocesanal administration more acceptable in an age when Normans were not at all popular in the areas of Wales which they controlled. However, the strategy seems to have been unsuccessful. Owain Gwynedd's open rebellion was in full swing and Geoffrey appears to have never even visited his bishopric. He died four years later, probably in London.