The historical Taliesin belongs to the later sixth-century world of British rulers and praise poetry. The ninth-century Historia Brittonum names him among notable poets of an earlier age. A group of poems traditionally associated with him praises Urien Rheged, Urien's son Owain and other rulers, including Gwallawg ap Lleenawg and Cynan Garwyn. The surviving texts are much later copies, and the attribution and dating of individual poems require care. They do not establish precise dates for his birth and death, nor prove that he personally rode in every battle he described.

The legendary life is preserved in the Taliesin story known from later prose and verse accounts. Ceridwen prepares a cauldron of inspiration for her son and assigns the boy Gwion Bach to stir it while Morda tends the fire. Three hot drops fall on Gwion's finger. When he puts it to his mouth, he gains the knowledge meant for another and understands that he must escape. In Lady Charlotte Guest's telling, the intended recipient is Afagddu, and Morfran is named separately as his brother. Other accounts identify them more closely, so the family relationships depend on the version.

Gwion's flight becomes a contest of transformations. He turns into a hare, fish and bird, while Ceridwen pursues him as a greyhound, otter and hawk. Finally he becomes a grain of wheat and she swallows him as a hen. She later gives birth to a beautiful child and, unable to kill him, casts him away. In Guest's telling, the child is wrapped in a leather bag and set adrift. The infant is found at Gwyddno Garanhir's weir by Elphin, and his radiant brow gives the story its explanation of the name Taliesin.

The miraculous bard then defends Elphin at the court of Maelgwn Gwynedd. His knowledge and poetry belong to that legendary contest, not to a documented performance by the historical sixth-century poet. Arthurian connections appear in other material. Preiddeu Annwn, preserved in the Book of Taliesin, speaks of Arthur's expedition to the Otherworld, while Geoffrey of Monmouth's Vita Merlini makes Taliesin a learned companion of Merlin. They speak to one another as different characters. The all-knowing voice of the later bard reaches beyond one court or one human lifetime, but those poetic claims are not autobiography.

The National Library of Wales dates the surviving Book of Taliesin, Peniarth MS 2, to the first half of the fourteenth century. Its single scribe brought together poems of different origins, including royal praise, religious learning and legendary voices. The manuscript's existence is historical; its attribution to Taliesin does not make every poem his personal composition. The historical bard and the later figure of rebirth, transformations and extraordinary memories can be studied together without making them one literal biography. Neither Gwion Bach nor Merlin is treated as an ordinary alternative name for every phase of the historical poet's life.