Phormio first appears in the historical record in 440 BC, when he shared with Thucydides, Hagnon, and others the command of the Athenian fleet in the later part of the Samian War. In 432 BC, he commanded a force of 1600 hoplites sent to assist in the siege of Potidaea. Phormio led his men up slowly from the side of the city that the Athenians had not yet surrounded and constructed a counter-wall to complete Potidaea's investment. After Potidaea was firmly besieged, Phormio led his men in a successful campaign against Athens' enemies in the Chalcidice, and in the next year he again led an army attacking the Chalcidians, this time alongside Perdiccas II, king of Macedon.