Cacamwri first appears in the Welsh prose tale Culhwch and Olwen, the earliest extended Arthurian story, preserved in the Red Book of Hergest and the White Book of Rhydderch. He is listed in the famous court catalogue, the roll call of Arthur's warriors and attendants that opens the tale. The catalogue describes him with a single vivid image: show him a barn, though there would be in it the work of fifty ploughs, and he would thrash away with an iron flail until the boards, the rafters, and the side beams would be no better off than the fine oats in the heap at the bottom. He is a force of nature, a human threshing machine whose strength is so excessive that the building itself is collateral damage.

Cacamwri shares a mother with Hygwydd, another of Arthur's servants whose duty was to carry the cauldron and place fire under it. The two brothers appear together throughout the tale, a matched pair of comic strongmen given the dirtiest and most dangerous jobs. When Arthur raids Ireland to take the cauldron of Diwrnach by force, Hygwydd hoists the cauldron onto his back while Cacamwri stands at his brother's side. The raid is a massacre: Llenlleawg the Irishman draws Caledfwlch and slays Diwrnach and his entire host, and Arthur's men escape with the cauldron full of Irish treasure.

The defining moment of Cacamwri's career comes during the hunt for Twrch Trwyth, the transformed Irish king who has become a monstrous boar. Arthur drives the beast into the Severn estuary, and a company of champions closes in. They seize the beast's feet and try to drown him, and in the struggle Mabon son of Modron snatches the razor from between the boar's ears. But before the comb can be seized, Twrch Trwyth touches bottom and bolts toward Cornwall. Cacamwri, as he is dragged up from the depths, has two quern stones dragged with him into the water, and the weight pulls him under. Osla Great-knife fares no better: his knife falls from its sheath, the sheath fills with water, and the waterlogged sheath drags him back down. The text implies they have drowned, yet Cacamwri appears alive later, suggesting Arthur's men hauled him back despite the stones.

The final appearance of Cacamwri and Hygwydd is the most humiliating. Arthur needs the blood of Orddu, the Very Black Witch, from her cave in the Uplands of Hell. Gwyn son of Nudd advises sending the brothers to fight her. They enter the cave, and Orddu immediately seizes Hygwydd by the hair, drags him to the floor, and begins to beat him. Cacamwri grabs the witch by her own hair and pulls her off, but she turns on him with a fury that only a woman interrupted can summon. She beats both brothers soundly, disarms them, and drives them out squealing and squalling. Arthur throws his knife Carnwennan from the entrance, striking Orddu until she is split in two like a pair of tubs. Caw of Pictland collects her blood, and Cacamwri's part in the quest is done.

Cacamwri's legacy is that of the comic strongman, the servant whose muscles are matched only by his misfortune. He is not a knight or a bard, but he is essential to the world of Culhwch and Olwen, where even the servants have superhuman abilities and the greatest strength is no match for a witch's temper or a millstone's pull. He is the ancestor of every strong but unlucky sidekick in comic literature, the man who does the heavy lifting and takes the heavy beating.