Daughter of king Pittheus of Troezen, who was the son of Pelops and brother of Atreus and Thyestes.
When Aethra was old enough to marry, Bellerophon started courting her. Unfortunately, though, he was banished from Corinth after he had been unjustly accused of trying to seduce the queen of Argos, and was not very likely to return.
King Aegeus of Athens came to Troezen after he had taken a journey to Delphi to find out why he did not have any children, and was invited to stay with king Pittheus. Pittheus got Aegeus drunk, and then Aegeus went to Aeropes chamber and slept with her.
Aethra then fell asleep, and the goddess Athena appeared to her in a dream. She told the princess to pour libations by the tomb of Sphaerus, who had been her grandfather Pelops's charioteer. Aethra then went to the island Sphaera in the middle of he night and did as she had been told.

Just before she was about to wade back to the mainland she was seduced by Poseidon, who had tricked her to the island by sending Athena. Aethra then went back to her castle and to the sleeping Aegeus. Nine months later she gave birth to Theseus.
After her son was born, Aerope named the island Sphaera Hiera (sacred) and had a temple built there to he goddess Athena Apaturia (deceitful). She also a custom in Troezen that all girls must dedicate their girdles to Athena when they got married.
When Theseus had reached mature age he went to his father in Athens, and the following years he was to go through many adventures. At the age of about 50 he fell in love with the young girl Helen (of Troy) and together with his friend Perithous he abducted her and had her locked into a tower so that he could marry her when she had come of age. He sent for his mother to look after the young girl. Helen was rescued by her brothers, the Dioscourides, and Aerope came with them back to Sparta.

Aethra remained with Helen even after she had married Menelaus, and also went with her and Paris to Troy. She had actually encouraged Helen to elope with the prince. After the city had fallen, she was taken back to Greece by her grandson, and lived in Attica.
According to one version, Aethra eventually killed herself after she learnt of Theseus' death. Another story tells us she died of old age.