Born at Crepy-en-Valois and related to Matilda, wife of William the Conqueror, at whose court he was educated. He became in 1072, on his father's death, the owner of considerable territory round Amiens and in the Vexin. He held these lands as a vassal of Philip I of France (1060–1108), who tried unsuccessfully to annex some of these lands. Simon wished to be a monk, but William wanted him to marry his daughter Adela. Simon went to Rome, ostensibly to see if the proposed marriage was within the prohibited degrees (as William's own marriage to Matilda had been), and on the way he took the monastic habit at the abbey of Saint-Claude at Condat (Cantal), and for some time lived as a hermit in the Jura.
His becoming a monk did not mean that he was without influence: on the contrary he was in considerable demand as a mediator and negotiator. St Hugh of Cluny recovered lands from the king of France through him; he also mediated in the quarrels between William and his sons; Pope Gregory VII had dealings through him with Robert Guiscard and retained him at Rome as an adviser from 1080. There Simon died, after receiving the Last Sacraments from Gregory. He was buried at St Peter's in Rome; the abbey of Saint–Claude claimed some of his relics later. Feast: 30 September.
Contemporary eulogies and an anonymous early Life are in AA.SS., s.d.See also Bibl. SS., xi. 1179–80 and B.L.S., xi. 283–4.