Critias gave an account of his ancestry which was later recorded in Plato's Timaeus. Critias's great-grandfather, Dropidas, was an intimate friend of Solon. Dropidas's son, also named Critias, was the grandfather and namesake of the author Critias.

Critias was once a student of Socrates. The two had a strained relationship. However, it is said that Critias was the one who saved Socrates from persecution during the terror of the Thirty Tyrants. However, Critias was very greedy, something that Socrates did not approve of.

After the fall of Athens to the Spartans, Critias, as one of the Thirty Tyrants, blacklisted many of its citizens. Most of his prisoners were executed and their wealth confiscated.

Critias was killed in a battle near Piraeus, the port of Athens, between a band of pro-democracy Athenian exiles led by Thrasybulus and members and supporters of the Thirty, aided by the Spartan garrison. In the battle, the exiles put the oligarchic forces to flight, ending the rule of the Thirty.