Aegisthus is the son of Thyestes and his own daughter Pelopia. Thyestes' brother Atreus killed Thyestes' sons, and served them to him at a banquet. Thyestes, seeking vengeance, consulted an oracle. The oracle said that Thyestes should have a child with his own daughter, Pelopia, who lived in Threspotia. Thyestes did as he was told. Pelopia, who was now pregnant, stayed in Threspotia, and Thyestes went to Lydia.
In Mycenae, a terrible famine was raging as a result of Atreus' crime. He consulted an oracle, which told him to bring Thyestes back to Mycenae. While searching for him, Atreus came to Threspotia. There, he saw the beautiful Pelopia, and asked for her to be his wife. The king agreed, and Pelopia returned to Mycenae with Atreus. When her child was born, she had him exposed, but he was rescued by some shepherds. When Atreus found out about the boy, named Aegisthus, he had him brought to the palace and raised as one of his sons.
Eventually, Thyestes was captured and brought to Mycenae. Atreus had him placed in a cell, and ordered Aegisthus to kill him. When the boy approached, Thyestes recognized his sword, which he had given to Pelopia years before. Aegisthus consulted his mother Pelopia, to see if this was true. When Pelopia realized that she had slept with her own father, she killed herself. Aegisthus then freed his father Thyestes and killed his step-father Atreus.
Thyestes was now the king of Mycenae, but not for long. The sons of Atreus, Agamemnon and Menelaus, returned and reclaimed the throne. Agamemnon became the king of Mycenae, and Menelaus the king of Sparta. Thyestes and Aegisthus were forced to flee.
Once Agamemnon sailed to Troy to fight in the Trojan War, Aegisthus returned to Mycenae and seduced Agamemnon's wife, Clytemnestra. Upon his return from Troy after ten years, the two lovers killed Agamemnon. Aegisthus ruled Mycenae for seven years. Eventually, Agamemnon's son returned from exile and killed both Aegisthus and Clytemnestra.