He was the son of William Wynell de Wenlock, commonly called William Wenlock, knight of the shire for Bedfordshire in 1404, by his wife Margaret Breton, an heiress of Houghton Conquest in Bedfordshire.
John Wenlock took part in the invasion of France under Henry V of England, and on 16 August 1421 he received a grant of lands in the bailiwick of Gisors in Normandy, and shortly after, in April 1422, is styled constable of Vernon.