Aeacus was the son of Zeus and Aegina, the daughter of the river god Asopos. He had a son, Phocus, with the Nereid Psamathe before marrying Endeis and having two more sons, Telamon and Peleus.
Zeus carried Aegina away to escape her parents' wrath, and the island he brought her to was later named Aegina as well. When Aeacus was born, there were no inhabitants; Zeus then created man either from the earth or from ants. Aeacus then became ruler of the newly inhabited land. He was renowned throughout all of Greece for his justice and piety, and was often called on to settle disputes among gods and men.
When Greece experienced a drought, the oracle of Delphi declared the drought wouldn't stop until Aeacus prayed to the gods for the drought to end. Aeacus prayed, and thanked the gods for ending the drought by building a temple for Zeus on Mount Panhellenion. Afterwards, the Aeginetians built a sanctuary named Aeaceum.
Phocus was Aeacus' favorite son, which caused his new family (Endeis, Telamon, and Peleus) to resent Phocus. The family conspired to have Phocus killed, and Telamon invited Phocus to participate with him in a pentathlon. During the pentathlon, Telamon threw a discus which hit and killed Phocus. Telamon and Peleus attempted to hide the body, but were discovered. Aeacus then exiled the two brothers from Aegina.
After his death, Aeacus became one of the three judges in Hades (the other two being Rhadamanthys and Minos).