Son of a French count; came to England in 1230, where he inherited from his grandmother the earldom of Leicester; attached to Henry III., and married to the king's sister, he was sent to govern Gascony in 1248; returned in 1253, and passed over to the side of the barons, whom he ultimately led in the struggle against the king; after repeated unsuccessful attempts to make Henry observe the Provisions of Oxford, Simon took arms against him in 1263; the war was indecisive, and appeal being made to the arbitration of Louis the Good, Simon, dissatisfied with his award, renewed hostilities, defeated the king at Lewes, and taking him and his son prisoner, governed England for a year (1264-65); he sketched a constitution for the country, and summoned the most representative parliament that had yet met, but as he aimed at the welfare of not the barons only, but the common people as well, the barons began to distrust him, when Prince Edward, having escaped from captivity, joined them, and overthrew Simon at Evesham, where he was slain (1206?-1265).