Danaus was born in Memphis, the ancient capital of Egypt, the son of Belus and Achiroe, also called Anchinoe, a Naiad nymph of the river Nilus. His father Belus was the son of Poseidon, the god of the sea and earthquakes, and he ruled over a vast territory that included Assyria, Egypt, Libya, and Arabia. Danaus had a twin brother named Aegyptus, and when their father divided the inheritance between them, Danaus received Libya, the land of the west, while Aegyptus received Arabia and later conquered Egypt, renaming it after himself. The two brothers raised their children in parallel. Aegyptus fathered fifty sons by various wives and nymphs. Danaus fathered fifty daughters, the Danaids, by an equally varied brood of mothers. The brothers' rivalry began in the cradle and would end in blood.

The crisis came when Aegyptus proposed that all fifty of his sons should marry all fifty of Danaus's daughters. Danaus suspected the truth: Aegyptus intended to use the marriages as a pretext to seize Libya, to kill Danaus and his daughters, and to unite both kingdoms under his own house. Danaus refused. Under the advice of Athena, the grey-eyed goddess of wisdom and war who had sprung fully armed from the head of Zeus, Danaus built the first ship ever made. It was a vessel of extraordinary design, large enough to carry fifty women and their possessions across the open sea. He gathered his daughters at Chemmis in Egypt, loaded the ship with their dowries and their hopes, and set sail for Greece.

The voyage was not without incident. They stopped at Rhodes, where Danaus founded the temple of Athena Lindia and dedicated a statue to the goddess who had saved them. Then they continued to the coast of the Peloponnese and landed at Apobathmi, the landing place, on the shore of Argos. At that very moment, Gelanor, the king of Argos, was holding an assembly on the seashore. A wolf suddenly attacked an ox nearby, and the Argives interpreted this as an omen that the stranger would conquer them. Gelanor, a prudent man, surrendered the kingdom without a fight. Danaus became king of Argos, and he claimed descent from Io, the Argive princess who had been transformed into a white heifer by Zeus and had wandered all the way to Egypt. The Argives, who valued their autochthonous freedom, accepted him as one of their own, and Danaus gave his name to the inhabitants, who were called Danai ever after.

But Aegyptus was not finished. He gathered his fifty sons, equipped them with an army, and sent them to Argos with a single command: do not return until Danaus is dead or the marriages are agreed. The sons of Aegyptus arrived at Argos and laid siege to the city. Danaus, seeing that the Argives would suffer for his quarrel, agreed to the marriages. A great wedding feast was held, and on a single night all fifty couples were married. But Danaus had prepared his daughters for treachery with treachery of his own. He gave each of the Danaids a dagger and commanded them to cut off their husbands' heads after the men had fallen asleep. Forty-nine of the daughters obeyed. They killed their bridegrooms in the darkness and brought the severed heads to their father as proof. Only Hypermnestra, the eldest, refused. Her husband was Lynceus, and he had respected her wish to remain a virgin on their wedding night, so she spared him and helped him escape to safety. The heads of the murdered sons were buried at Lerna, and the bodies elsewhere, while Athena and Hermes, the messenger god, purified the ground at Zeus's command.

Danaus ruled Argos for many years after the massacre. He held funeral games for the dead, or in some accounts he simply continued to govern the city with the same prudence that had won it for him. He eventually found new husbands for his daughters, offering them as prizes in a footrace to the suitors who could catch them. The winners married the Danaids and founded the noble families of Argos. But Danaus could not escape the curse of his own command. Lynceus, the only surviving son of Aegyptus, eventually returned to Argos and killed Danaus in revenge, or in some versions he simply succeeded him as king after Danaus died of old age. Through Hypermnestra and Lynceus, the line continued to Abas, Acrisius, and Danae, the mother of Perseus, the hero who cut off the head of the Gorgon Medusa. From Perseus the line stretched to Heracles, the greatest hero of Greece, and to Eurystheus, the king who assigned him his twelve labours. The Danaids themselves did not escape punishment. In the underworld, they were condemned to carry water in bottomless jars or sieves, forever trying to fill a leaking cistern, a torment that symbolised the futility of their crime.