Cleisthenes was a wealthy aristocrat from the Alcmeonid family, many of whom had been distinguished in Athenian politics for their opposition to tyranny. In the 7th century, one of his ancestors had been responsible for killing the tyrant Cylon, and his father Megacles was responsible for banishing the tyrant Pisistratus.

Cleisthenes was one of the people responsible for overthrowing the tyrant Hippias, son of Pisistratus. Afterwards, Cleisthenes was involved in major democratic reforms of Athens (ca. 508). In particular, he reorganized Athens into 10 demes or tribes that were artificial geographical divisions (like US voting districts) to prevent family-based factions from taking over elections. Next, he introduced the system of selecting the boule, or legislative body of Athens by lot, with 50 men chosen at random from each deme by placing the names of all eligible men from the deme on potsherds in a large urn and having a blindfolded priestess of Delphi select 50 names, a process designed to avoid bribery and demagoguery. Finally, he may have been responsible for the introduction of ostracism, a process by which the Assembly could banish for a fixed period someone who they thought was growing so powerful as to be a potential tyrant.