An Athenian, the son of a tanner, and said himself to have exercised that trade. Of extraordinary impudence and little courage, slow in the field, but forward and noisy in the assembly, corrupt, but boastful of integrity, and supported by a coarse but ready eloquence, he gained such consideration by flattering the lower orders that he became the head of a party. By an extraordinary train of circumstances he came off victorious in the affair of Sphacteria, the Athenian populace having chosen him one of their generals. Elated upon this with the idea that he possessed military talents, he caused himself to be appointed commander of an expedition into Thrace. He was slain in a battle at Amphipolis against Brasidas, the Spartan general, B.C. 422.
It is probably unfortunate in the interest of historical truth that the accounts we have of Cleon's personality exist only in the writings of Thucydides and a partisan play, The Knights, of Aristophanes, both of whom were violently prejudiced against Cleon, the former personally and the latter politically.
(Harpers Dictionary of Classical Antiquities, 1898.)