Le Gros was probably descended from William Gross, a Norfolk landowner mentioned in the Domesday Book. However, only one previous member of the family, Oliver Groos, knight of the shire in 1419, had sat in Parliament, and his immediate progenitors were not sufficiently prominent to be members of the Norfolk bench.
Le Gros’s maternal grandfather, Sir Charles Cornwallis, had been returned for Norfolk in 1604. His wife’s family, the Knyvetts, were prominent in Norfolk politics and were connected by marriage to Cornwallis’ 1604 colleague, (Sir) Nathaniel Bacon, whom Le Gros’s paternal grandfather appointed supervisor of his will.
Le Gros was added to the Norfolk bench in June 1624, and the following year stood for the county, only to be defeated at the election held on 16 April.
Le Gros presumably owed his election for Orford in 1626 to his friend, the 1625 Member, Sir William Withypoll*, for whom he acted as surety.
