Anna first appears by name in Geoffrey of Monmouth's Historia Regum Britanniae, composed around 1136, the book that created the Arthurian legend as the medieval world would know it. In Geoffrey's account, Uther Pendragon fell in love with Igraine, the wife of Duke Gorlois of Cornwall, and with Merlin's magic he entered her bed disguised as her husband. From that union came two children: Arthur, the son who would draw the sword from the stone, and Anna, the daughter whose birth Merlin had already foretold. When Uther's brother Ambrosius lay dying, a comet appeared in the shape of a dragon with two beams of light streaming from its mouth. Merlin prophesied that one beam signified Arthur, while the other signified a daughter whose descendants would rule Britain in a line of seven kings.

Uther married Anna to Lot of Lothian while Arthur was still a boy, a political alliance to secure the northern frontier against the Saxons. Lot was a powerful warlord, one of three brothers who ruled the north, and his support was essential to the Pendragon cause. In Geoffrey's telling, Anna became the mother of Gawain and Mordred, the two brothers who would dominate the later books of the Historia. Gawain grew to be Arthur's most trusted warrior, the knight who led armies and fought at the king's right hand, while Mordred became the traitor who seized the throne and brought about the final battle at Camlann. The chronicle tradition made the matter plain: Anna was the mother of both, and the tragedy of Arthur's house began in her womb.

The most vivid story of Anna's life appears in the Latin romance De Ortu Waluuanii, composed in the late thirteenth century, which tells the early life of Gawain. In this tale, Lot came to Uther's court as a young hostage from Norway, and Anna fell in love with him in secret. Their affair produced a child before they were married, and Anna, fearing her father's wrath, hid her pregnancy from everyone. When the boy was born she placed him in a casket with her royal signet ring and a parchment attesting to his parentage, and sent him away with a merchant. The child was found by a fisherman, grew to manhood in ignorance of his birth, and came to Arthur's court as the Knight of the Surcoat, proving his identity when his uncle recognised the ring. It is the only extended narrative in which Anna acts as a protagonist: a princess who risked everything for love and trusted her son's fate to the sea.

In Welsh tradition, Anna was identified with Gwyar, the mother of Gwalchmei, the Welsh name for Gawain. The Welsh Bruts, adaptations of Geoffrey's Latin chronicle, retained the name Anna while also introducing Gwyar as Arthur's sister and the wife of Lleu, the Welsh form of Lot. Some texts gave Anna a first husband, Emyr Llydaw, the Emperor of Brittany, by whom she was mother of Hoel, the king who became Arthur's closest ally in the continental wars. This doubling of names suggests that Anna was not merely Geoffrey's invention but rested on older Welsh tradition, a sister of Arthur whose memory was too strong to be erased even when foreign names were imposed upon it.

The later Arthurian romances, however, had no place for Anna. As the legend moved from chronicle into chivalric romance, the sober sister of the king was replaced by the more dramatic Morgause, the enchantress who slept with her own brother and bore Mordred in incest. Morgause took Anna's husband, her sons, and her place in the story, while Anna herself was forgotten. The Scottish chronicler John Fordun, writing around 1385, tried to reclaim her by making her Arthur's aunt rather than his sister, and by suggesting that Mordred's rebellion was justified because Anna's line held the legitimate claim to the throne. Hector Boece expanded this idea in 1527, turning Anna into Cristina and making Mordred the rightful heir passed over for Arthur's illegitimate crown. But these were political arguments, not stories, and Anna the woman remained silent.