According to Hesiod, Geryon had one body and three heads, whereas the tradition followed by Aeschylus gave him three bodies. A lost description by Stesichoros said that he had six hands and six feet and was winged; some mid-6th century BC Chalcidian vases portray Geryon as winged. Some accounts state that he had six legs as well, while others state that the three bodies were joined to one pair of legs. Apart from these bizarre features, his appearance was that of a warrior.

He owned a two-headed hound named Orthrus, the brother of Cerberus, and a herd of magnificent red cattle that were guarded by Orthrus and by a herder named Eurytion (son of Erytheia).

The Tenth Labour of Heracles. In the fullest account, in the Bibliotheke of Pseudo-Apollodorus, Heracles was required to travel to Erytheia to obtain the Cattle of Geryon as his tenth labour. On the way he crossed the Libyan desert and, so frustrated by the heat, shot an arrow at Helios, the Sun. Helios, in admiration of his courage, gave Heracles the golden cup he used to sail across the sea from west to east each night, and Heracles used it to reach Erytheia — a favourite motif of the vase-painters that undercuts any literal geography for the "red island" of the sunset.

When Heracles landed he was at once confronted by the two-headed dog Orthrus, which he killed with a single blow of his olive-wood club; Eurytion the herdsman came to assist and was dealt with the same way. Hearing the commotion, Geryon sprang into action carrying three shields, three spears, and wearing three helmets. He pursued Heracles at the River Anthemus but fell to an arrow dipped in the venomous blood of the Lernaean Hydra, shot so forcefully that it pierced his forehead — "and Geryon bent his neck over to one side, like a poppy that spoils its delicate shapes, shedding its petals all at once."

Heracles then had to herd the cattle back to Eurystheus. In Roman versions, on the Aventine Hill, Cacus stole some of the cattle as Heracles slept, making them walk backwards to leave no trail, a repetition of the trick of the young Hermes; Heracles discovered the theft (in some tellings the lowing cattle called to one another, in others Cacus' sister Caca revealed his cave) and killed Cacus, founding an altar where the Forum Boarium, the cattle market, was later held. To annoy Heracles, Hera sent a gadfly to scatter the herd and a flood to bar his crossing, which he overcame by piling stones to shallow the river; when he reached the court of Eurystheus, the cattle were sacrificed to Hera. In the Aeneid, Vergil may have based the triple-souled Erulus, king of Praeneste, on Geryon, and Hercules' conquest of Geryon is mentioned in Book VIII.

Stesichorus' Account. The poet Stesichorus wrote a poem, "Geryoneis," in the sixth century BC, apparently the source of the Bibliotheke account and containing the first reference to Tartessus. From fragmentary papyri found at Oxyrhynchus, it appears Stesichorus may have introduced a character, Menoites, who reported the theft of the cattle to Geryon. Geryon then had an interview with his mother Callirrhoe, who begged him not to confront Heracles, and they expressed doubt over whether he would prove immortal. The gods met in council, where Athena warned Poseidon that she would protect Heracles against Poseidon's grandson Geryon. Denys Page observed that representations of the Geryon episode in vase-painting increased from the mid-sixth century, suggesting the "Geryoneis" provided the impetus.

Pausanias' Account. In his Description of Greece, Pausanias mentions that Geryon had a daughter, Erytheia, who had a son with Hermes, Norax, the founder of the city of Nora in Sardinia.

The Geryon of Dante's 14th-century epic Inferno bears no resemblance to earlier writings. Here Geryon has become the Monster of Fraud, a beast with the paws of a bear or lion, the body of a wyvern, and a scorpion's poisonous sting at the tip of his tail, but with the face of an "honest man" (similar to a manticore).

He dwells in the shadowed depths below the cliff between the seventh and eighth circles of Hell (violence and simple fraud). Geryon rises from the pit at Virgil's call, and — to Dante's horror — Virgil requests a ride on the creature's back; the two then board him as he glides in slow descending circles around the waterfall of the river Phlegethon down to the Circle of Fraud.

The myth of Geryon is linked to the building of the nations of Spain and Portugal, since he was considered an inhabitant of the Iberian Peninsula. Medieval authors such as the bishop of Girona Joan Margarit i Pau (1422–1484) and the bishop of Toledo Rodrigo Jiménez de Rada tried to legitimise the resistance of Geryon against the Greek invader.

The Estoria de España of Alfonso X of Castile tells how Hercules killed the giant Geryon, cut off his head, and ordered a tower built on it to mark his victory; the Tower of Hercules in Coruña, Spain, is a working lighthouse rebuilt on a Roman one. The Portuguese friar Bernardo de Brito considered the monster a historical invader who ruled despotically over the descendants of Tubal.