In 437/6 BC, he led the settlers who founded the city of Amphipolis in Thrace; in the Peloponnesian War, he served as an Athenian general on several occasions, and was one of the signers of the Peace of Nicias and the alliance between Athens and Sparta.
In 411 BC, during the oligarchic coup, he supported the oligarchy and was one of the ten commissioners (probouloi) appointed to draw up a new constitution.
Hagnon's first appearance in the historical records is in 440 BC, when he is mentioned as one of the generals bringing 40 triremes from Athens to reinforce Pericles at the Siege of Samos.
Two previous attempts to found an Athenian colony on this valuable location (the site was desirable both because of its strategic position on the trade routes between the Hellespont and mainland Greece and because it was the primary outlet for trade from the wealthy Strymon valley)[3] had been defeated by hostile native populations, but Hagnon, leading a multinational force of settlers, defeated the Edonians who held the location and founded the city of Amphipolis on an island in the river.
[7] In 411 BC, when revolutionary forces at Athens took advantage of the disorder in the wake of the Sicilian Expedition to overthrow the Athenian democracy and replace it with an oligarchy, Hagnon was a member of the government of 400 oligarchs that was established, and served as one of the ten commissioners charged with drafting a new constitution.