The first of Hercules' twelve labours, set by his cousin King Eurystheus, was to slay the Nemean lion.

According to one version of the myth, the Nemean lion took women as hostages to its lair in a cave near Nemea, luring warriors from nearby towns to save the damsel in distress. After entering the cave, the warrior would see the woman (usually feigning injury) and rush to her side. Once he was close, the woman would turn into a lion and kill the warrior, devouring his remains and giving the bones to Hades.

Hercules wandered the area until he came to the town of Cleonae. There he met a boy who said that if Hercules slew the Nemean lion and returned alive within thirty days, the town would sacrifice a lion to Zeus, but if he did not return within thirty days or he died, the boy would sacrifice himself to Zeus. Another version claims that he met Molorchos, a shepherd who had lost his son to the lion, saying that if he came back within thirty days, a ram would be sacrificed to Zeus. If he did not return within thirty days, it would be sacrificed to the dead Hercules as a mourning offering.

While searching for the lion, Hercules fletched some arrows to use against it, not knowing that its golden fur was impenetrable; when he found and shot the lion and firing at it with his bow, he discovered the fur's protective property when the arrow bounced harmlessly off the creature's thigh. After some time, Hercules made the lion return to his cave. The cave had two entrances, one of which Hercules blocked; he then entered the other. In those dark and close quarters, Hercules stunned the beast with his club and, using his immense strength, strangled it to death. During the fight the lion bit off one of his fingers. Others say that he shot arrows at it, eventually shooting it in the unarmored mouth. After slaying the lion, he tried to skin it with a knife from his belt, but failed. He then tried sharpening the knife with a stone and even tried with the stone itself. Finally, Athena, noticing the hero's plight, told Hercules to use one of the lion's own claws to skin the pelt. Others say that Hercules' armor was, in fact, the hide of the lion of Cithaeron.