He was son of Zeus and Aegina, a daughter of the river-god Asopus. He was born in the island of Oenone or Oenopia, to which Aegina had been carried by Zeus to secure her from the anger of her parents, and whence this island was afterwards called Aegina. According to some accounts Aeacus was a son of Zeus and Europa. Some traditions related that at the time when Aeacus was born, Aegina was not yet inhabited, and that Zeus changed the ants (μύρμηκες) of the island into men (Myrmidons) over whom Aeacus ruled, or that he made men grow up out of the earth. Ovid, on the other hand, supposes that the island was not uninhabited at the time of the birth of Aeacus, and states that, in the reign of Aeacus, Hera, jealous of Aegina, ravaged the island bearing the name of the latter by sending a plague or a fearful dragon into it, by which nearly all its inhabitants were carried off, and that Zeus restored the population by changing the ants into men.
These legends are nothing but a mythical account of the colonization of Aegina, which seems to have been originally inhabited by Pelasgians, and afterwards received colonists from Phthiotis, the seat of the Myrmidons, and from Phlius on the Asopus. Aeacus while he reigned in Aegina was renowned in all Greece for his justice and piety, and was frequently called upon to settle disputes not only among men, but even among the gods themselves. He was such a favourite with the latter, that, when Greece was visited by a drought in consequence of a murder which had been committed, the oracle of Delphi declared that the calamity would not cease unless Aeacus prayed to the gods that it might. Aeacus prayed, and it ceased in consequence. Aeacus himself showed his gratitude by erecting a temple to Zeus Panhellenius on mount Panhellenion, and the Aeginetans afterwards built a sanctuary in their island called Aeaceum, which was a square place enclosed by walls of white marble. Aeacus was believed in later times to be buried under the altar in this sacred enclosure.
A legend preserved in Pindar relates that Apollo and Poseidon took Aeacus as their assistant in building the walls of Troy. When the work was completed, three dragons rushed against the wall, and while the two of them which attacked those parts of the wall built by the gods fell down dead, the third forced its way into the city through the part built by Aeacus. Hereupon Apollo prophesied that Troy would fall through the hands of Aeacus's descandants, the Aeacidae.
Aeacus was also believed by the Aeginetans to have surrounded their island with high cliffs to protect it against pirates. Several other incidents connected with the story of Aeacus are mentioned by Ovid. By Endeïs Aeacus had two sons, Telamon and Peleus, and by Psamathe a son, Phocus, whom he preferred to the two others, both of whom contrived to kill Phocus during a contest, and then fled from their native island.
After his death Aeacus became one of the three judges in Hades, and according to Plato especially for the shades of Europeans. In works of art he was represented bearing a sceptre and the keys of Hades. Aeacus had sanctuaries both at Athens and in Aegina, and the Aeginetans regarded him as the tutelary deity of their island.
In The Frogs (405 BC) by Aristophanes, Dionysus descends to Hades and announces himself as Heracles. Aeacus laments Heracles's theft of Cerberus and sentences Dionysus to Acheron and torment by hounds of Cocytus, Echidna, the Tartesian eel, and Tithrasian Gorgons.
Alexander the Great traced his ancestry (through his mother) to Aeacus.