In Greek Mythology, he was the only child of Thyestes to survive to adulthood. Thyestes was the son of Pelops, and the brother of Atreus. Thyestes fell in love with Atreus' wife, Aerope. He succeeded in making her false to her wedding vows, but his brother found out. Atreus murdered Thyestes children and fed them to him. Aegisthus was not fed to his father because he had not been born yet.

Aegisthus becomes Clytemnestra's lover, after her husband, Agamemnon, son of Atreus, sacrificed Iphigenia to Artemis to ensure safe passage to Troy. When Agamemnon returns from the Trojan War, Aegisthus and Clytemnestra murder him. Aegisthus eventually dies at the hand of Orestes, son of Agamemnon and Clytemnestra.