Hesiod writes that the Nemean lion is the offspring of Orthus and an ambiguous "she," usually understood as referring to either the Chimera, Echidna, or possibly Ceto. Depending on its parentage the lion is brother to the Theban Sphinx and shares the lineage of other monsters bound up in the Heracles myth, such as Cerberus and the Lernaean Hydra. Apollodorus, however, depicts the lion as the offspring of Typhon.

According to Hesiod the lion was raised by Hera and sent to terrorize the hills of Nemea. In another tradition, told by Aelian (citing Epimenides) and Hyginus, the lion sprang from the moon-goddess Selene, who threw it from the Moon at Hera's request onto Mount Apesas. Hera allowed the beast to hunt within her lands, knowing it would cause more trouble for Heracles. In the Bibliotheca, Photius wrote that the dragon Ladon, guardian of the golden apples, was the Nemean lion's brother.

The first of Heracles' twelve labours, set by King Eurystheus, was to slay the Nemean lion. Heracles wandered the area until he came to the town of Cleonae, where he met a boy who promised that if Heracles slew the lion and returned alive within thirty days the town would sacrifice a lion to Zeus; if he failed or died, the boy would sacrifice himself to Zeus. Another version has him meet Molorchos, a shepherd who had lost his son to the lion, who agreed to sacrifice a ram to Zeus if Heracles returned within thirty days, or to the dead Heracles as a mourning offering if he did not.

Searching for the lion, Heracles shot at it with arrows, not knowing its golden fur was impenetrable; the arrows bounced harmlessly off its thigh. He then drove the lion back to its cave, which had two entrances. Blocking one and entering by the other so the beast could not escape, he stunned it with his club in the dark, close quarters and finally killed it by strangling it with his bare hands.

After the kill Heracles tried to skin the lion with a knife, then sharpened the knife with a stone and even tried the stone itself, all in vain. Athena (or, in Theocritus, simply "some god") inspired him to use one of the lion's own claws to cut the pelt. When Heracles returned on the thirtieth day with the carcass on his shoulders, Eurystheus was amazed and terrified; he forbade Heracles from entering the city again and had a bronze jar buried in the earth in which to hide whenever the hero returned. Thereafter Eurystheus communicated with Heracles only through an intermediary, Copreus, and sent him off to destroy the Lernaean Hydra.

According to Alexander of Myndus, Heracles was helped in this labour by an Earth-born serpent that followed him to Thebes and settled at Aulis, later identified with the water snake that devoured the sparrows and was turned to stone in the prophecy about the Trojan War.

Heracles wore the Nemean lion's coat after killing it, as it was impervious to the elements and all but the most powerful weapons. Others say his armour was instead the hide of the Lion of Cithaeron, and Euripides states the pelt came from a lion slain within Zeus' grove. This element of the myth may have been created post-Homerically, since Stesichorus was the first to give Heracles his characteristic lion-skin costume; earlier he was shown dressed like an ordinary warrior. As March supposes, the wearing of the fur must have followed the lion's gaining invulnerability to mortal weapons. The lion could be read as a symbol of death, so that by skinning and wearing the pelt Heracles took away the threat of death and turned it into deliverance from it.

While the Homeric tradition does not say what becomes of the skin at Heracles' death on the pyre, most ancient artistic depictions show him wearing the pelt as he is set aflame. The symbolism of the pelt was also used to link others to Heracles, such as Alexander the Great, depicted on the Alexander Sarcophagus wearing a lion's pelt on his head as he rides into combat.

To celebrate Heracles' feat, Zeus used the Nemean lion to create the constellation Leo. The Nemean Games were later founded in honour of the labour, and, like Heracles after his victory, the victors crowned themselves with a garland of wild celery.

Most ancient depictions show the Nemean lion in battle with Heracles, who appears in various stages of dress or nudity, the two typically wrestling one another. Images in which the lion is neither locked in combat with Heracles nor dead and worn by the hero are comparatively rare. Artistic treatments span from ancient Greek pottery through later sculpture, with the impervious pelt remaining the enduring visual emblem of the myth.