Cleisthenes was a noble Athenian of the accursed Alcmeonidate family. He is credited with reforming the constitution of ancient Athens and setting it on a democratic footing in 508 BCE. He was related to the tyrant Cleisthenes of Sicyon, through the latter's daughter Agariste and her husband Megacles. With help from the Alcmaeonidae (Cleisthenes' genos, "clan") and the Spartans, he was responsible for overthrowing Hippias, the son of the tyrant Pisistratus. After the collapse of the Pisistratid tyranny, Isagoras and Cleisthenes were in rivalry for power, but Isagoras won the upper hand by becoming archon in 507/8. Cleisthenes responded by gaining support from the hitherto unrepresented masses. Isagoras appealed to the Spartan king Cleomenes I to help him expel Cleisthenes, he did so on the pretext of the Alcmaeonid curse. Consequently, Cleisthenes left Athens as an exile, and Isagoras was singly unrivalled in power inside the city, and attempted to establish an oligarchy. Therefore, he set about uprooting hundreds of people from their homes on the affectation that they too were cursed, and attempted to dissolve the council. However, the council resisted, and the Athenian people declared their support in favour of it. Hence Isagoras and his supporters were forced to flee to the Acropolis, and were besieged there for two days, until on the third, it was agreed that a truce be called so that Cleomenes and his men could be released.

Cleisthenes was subsequently recalled, along with the hundreds of exiles, and he assumed leadership of Athens. After this victory Cleisthenes began to reform the government of Athens. He eliminated the four traditional tribes, which were based on family relations and had led to the tyranny in the first place, and organized citizens into ten tribes according to their area of residence (their deme). Most modern historians suppose there were 139 demes (this is still a matter of debate), organized into thirty groups called trittyes ("thirds"), with ten trittyes divided among three regions in each deme (a city region, asty; a coastal region, paralia; and an inland region, mesogeia). He also established legislative bodies run by individuals chosen by lot, rather than kinship or heredity. He reorganized the Boule, created with 400 members under Solon, so that it had 500 members, 50 from each tribe. The court system (Dikasteria - jury courts) was re-organized and had from 201-5001 jurors selected each day, up to 500 from each tribe. It was the role of the Boule to propose laws to the assembly of voters, who convened in Athens around forty times a year for this purpose. The bills proposed could be rejected, passed or returned for amendments by the assembly.

Cleisthenes also seems to have introduced ostracism (first used in 487 BC), whereby the citizens voted to exile a citizen for 10 years. The initial trend was to vote for a citizen deemed a threat to the democracy e.g. by having ambitions to set himself up as tyrant. However, soon after, any citizen judged to have too much power in the city tended to be targeted for exile e.g. Xanthippus in 485/84 BC. Under this system, the exiled man's property was maintained, but he was not physically in the city where he could possibly create a new tyranny. Cleisthenes called these reforms isonomia ("equality under the law"), rather than democratia. Soon after his reforms, his life becomes a mystery since none of the ancient texts available to us mention him thereafter.