Merlin, Arthur's adviser, prophet and magician, is basically the creation of Geoffrey of Monmouth, who in his twelfth-century History of the Kings of Britain combined the Welsh traditions about a bard and prophet named Myrddin with the story that the ninth-century chronicler Nennius tells about Ambrosius (that he had no human father and that he prophesied the defeat of the British by the Saxons).

Geoffrey gave his character the name Merlin us rather than Merdinus (the normal Latinization of Myrddin) because the latter might have suggested to his Anglo-Norman audience the vulgar word "merde." In Geoffrey's book, Merlin assists Uther Pendragon and is responsible for transporting the stones of Stonehenge from Ireland, but he is not associated with Arthur. Geoffrey also wrote a book of "Prophecies of Merlin" before his History. The Prophecies were then incorporated into the History as its seventh book. These led to a tradition that is manifested in other medieval works, in eighteenth-century almanac writers who made predictions under such names as Merlinus Anglicus, and in the presentation of Merlin in later literature.

Merlin became very popular in the Middle Ages. He is central to a major text of the thirteenth-century French Vulgate cycle, and he figures in a number of other French and English romances. Sir Thomas Malory, in the Morte d'Arthur presents him as the adviser and guide to Arthur.

In the modern period Merlin's popularity has remained constant. He figures in works from the Renaissance to the modern period. In The Idylls of the King, Tennyson makes him the architect of Camelot. Mark Twain, parodying Tennyson's Arthurian world, makes Merlin a villain, and in one of the illustrations to the first edition of Twain's work illustrator Dan Beard's Merlin has Tennyson's face. Numerous novels, poems and plays center around Merlin. In American literature and popular culture, Merlin is perhaps the most frequently portrayed Arthurian character.