Aegyptus was born of divine blood on both sides. His father was Belus, the son of Poseidon, the god of the sea and earthquakes, who was described in Greek genealogy as a king of Egypt. His mother was Achiroe, also called Anchinoe, a Naiad nymph of the river Nilus, the great Egyptian waterway that flowed from the southern cataracts to the Mediterranean Sea. Aegyptus had a twin brother named Danaus, and together they inherited the kingdom their father had built. Belus divided the realm between them: Danaus received Libya, the land of the west, and Aegyptus received Arabia, the eastern peninsula. But Aegyptus was not content with his share. In the myth, he expanded his territory, conquered the kingdom of the Nile, and renamed it Egypt after himself. This is a legendary explanation of a place-name, not a demonstrated historical event.

The trouble began when the two brothers began to raise their children. Danaus fathered fifty daughters, the Danaids, by various wives and nymphs. Aegyptus fathered fifty sons by an equally varied brood of mothers: Argyphia, a woman of royal blood; Caliadne, a Naiad nymph; Tyria; Gorgo; Hephaestine; and an unnamed Phoenician woman, among others. The sons grew into warriors, and the daughters grew into women, and Aegyptus saw an opportunity. A marriage between all fifty sons and all fifty daughters became part of the dispute, a proposed union that would bind the two families together. But Danaus suspected the truth. He believed that Aegyptus intended to use the marriages as a pretext to overthrow him, to kill him and his daughters, and to seize Libya as he had already seized Egypt. 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 a ship and fled across the sea with his fifty daughters; Apollodorus credits him with being the first to do so. They sailed from Egypt, stopped at Rhodes to honour Athena Lindia with a monument, and finally landed at Apobathmi on the coast of Argos, the ancient Greek city in the Peloponnese. There Danaus 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, and he became king of Argos. The conflict followed him across the sea as the sons of Aegyptus came after the daughters and pressed their claim to marry them.

The sons of Aegyptus arrived at Argos and asked Danaus to end the hostility between their families. Danaus distrusted them and resented his exile, but 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 daughters at Zeus's command.

What became of Aegyptus himself is told only in fragments. The Greek traveller Pausanias, walking through Greece in the 2nd century CE, reported that the tomb of Aegyptus could be seen in a sanctuary of Serapis at Aroe, near Patrae in Achaea. The local tradition connected his arrival in the region with the disaster that had overtaken his sons and his fear of Danaus. The reported tomb formed part of the landscape through which later Greeks remembered the story. In other versions, Lynceus later killed Danaus in revenge and became king of Argos himself, founding the dynasty that would rule the city for generations. The name of Aegyptus, meanwhile, remained part of the Greek attempt to connect Egypt with its own legendary genealogies. The story must not be mistaken for a proven derivation of the country's name, and its tragedy is not the extinction of every son: Lynceus survived.