A manuscript narrating the foundation of St Andrew’s Priory, Northampton records that “duo fratres…Garnerius dictus le Ryche et Simon de Seynlyz filii Raundoel le Ryche” accompanied William “the Conqueror” to England.
He was created Earl of Huntingdon and Northampton in [1087/90] after his marriage, presumably de iure uxoris, although his late father-in-law's earldom must have been forfeited in  implying that a new grant would have been necessary. He witnessed a charter to Bath Abbey as "Earl Simon" in 1090. He built the castle of Northampton. “Symon et uxor mea Matildis” founded the St Andrew’s, Northampton by undated charter, subscribed by “…Johannis nepotis comitis…Symonis nepotis comitis, Warneri nepotis comitis…Petri nepotis comitis…”. "…Symonis comitis…" subscribed a charter dated 14 Sep 1101 under which Henry I King of England donated property to Bath St Peter.
A manuscript narrating the foundation of St Andrew’s Priory, Northampton records that Simon died “apud Caritatem” while returning from a journey to “terram sanctam” and was buried there. 1
SIMON DE ST. LIZ, said to be a son of Ranulph the Rich, a Norman, appears to have come to England early in the reign of William II. Presumably in consequence of his mariiage, he became Earl of HUNTINGDON and NORTHAMPTON after 1086 (for he is not named in Domesday Book) and in or before 1090, when he witnessed a charter to Bath Abbey as "Earl Simon." He witnessed another royal charter under the same designation a little later. He fought for William in Normandy in 1098, and was taken prisoner by Louis, son of the French King. On the accession of Henry I in 1100 he witnessed the charter of liberties issued by the King at his Coronation. He built the Castle of Northampton and founded or refounded the Priory of St. Andrew in that town, and made it dependent on the Cluniac house of La Charité-sur-Loire; this was probably in the time of William Rufus, but certainly before 1108, when he granted an ample charter to it in conjunction with Maud his wife. He was a benefactor also to Daventry Priory, and probably built St. Sepulchre's, Northampton, about this time. He went to Jerusalem cruce signatus, and returned safely, but setting out again he died on the way at the abovenamed Priory of La Charité, and was buried there. 2
Note the following shows Simon de Senlis as the third son of Landri de Senlis, seigneur de Chantilly, and his wife Ermengarde:
Simon de Senlis, I. du nom, troisieme fils de Landry, I. du nom, Seigneur de Chantilly & d'Ermengarde, son épouse, étant passé en Angleterre, le Roi Guillaume le Conquérant lui voulut faire épouser Judith sa niéce, fille de Mahaud, Comtesse d'Aumale, sa soeur utérine, veuve de Waleve [Waltheof], Comte de Hu[n]tingdon & de Northampton, qui le refusa, à cause qu'il étoit boiteux, comme remarque Ingulphe, Abbé de Croelaud, lequel ajoute en l'Histoire de la fondation de son Abbaye, que le Roi irrité contre elle de ce refus, la priva de ces deux Comtés, qu'il donna à Simon de Senlis, & le maria l'an 1010, avec Mahaud, fille aînée de la Comtesse & dudit Waleve. Selon le même Auteur, Simon de Senlis fit bâtir le Château de Northampton & le Monastere de S. André, & étant mort avant la Comtesse Mahaud sa femme, elle se remaria avec David, Roi d'Ecosse, laissant de son premier mariage deux fils & une fille: --1. Simon, qui suit; --2. Waleve, Abbé de Melrose; --3. Mahaud, femme de Robert, Seigneur de Wodhamuoter & de Dunmon. 3