Fleming was a descendant of Archembald le Fleming of Bratton Fleming, Devon, who was alive in 1087. Archembald's grandson, Archembald fitz Stephen le Fleming, came to Ireland with King Henry II of England in 1171 and participated in Hugh de Lacy's plantation of the kingdom of Kingdom of Mide. He was the great-great grandfather of Simon.
Simon was a member of the Irish Parliament of 1370. He became the lord of Slane in 1335 on the death of his father. In 1346 he served as Commissioner of the Peace in County Meath. He had been knighted before 1365.