In 431 BCE, Onomakles was assigned by the Athenian general Kleon to meet his informant Heremos and escort him to Mytilene in Lesbos for a mission. However, Onomakles and Heremos were both captured by the Spartan Army which had occupied the region surrounding Athens and were sent to the Mt. Pentelikos Marble Quarry. While Heremos was hanged to death, Onomakles was held captive in a cage. Sometime later, Onomakles was freed by the misthios Kassandra, whom Kleon had sent to initially free Heremos. After being carried to safety, Onomakles requested Kassandra to escort him to Marathon Beach, where his ship was docked.
After freeing his men and destroying the nearby Spartan ships, Onomakles thanked Kassandra for her efforts, remarking that she had earned the favor of "some powerful people" before returning to his ship and mission.