Hagnon held military command for Athens on several occasions, sharing in the command of the force that sailed against Samos in the Samian War of 440 BC and commanding a force that attempted to capture Potidaea in 430 BC. In 421 BC, he was one of the Athenian signers of the Peace of Nicias, the treaty, negotiated by an Athenian general with the same name as his father (no relation), that brought an end to the so-called Archidamian War, the first stage of the Peloponnesian War; he also signed the alliance between Athens and Sparta that was concluded in that same year.