His father and his elder brother Henry were killed at the Battle of Evesham in August 1265. The younger Simon had been slow to bring his forces from London, and had seen them and their banners captured by Prince Edward, who then used the banners to trick Simon's father. The younger Simon came to Evesham just in time to see his father's head atop a pike. The younger Simon tried to raise a rebellion in Lincolnshire, but this had petered out by Christmas.
He and his younger brother Guy escaped to France and Italy: in 1271 they discovered and murdered their cousin Henry of Almain (whom they blamed for the death of their father) at the church in Viterbo, for which they were excommunicated. Simon died later that year from Tuscan fever at Siena, "cursed by God, a wanderer and a fugitive".