 Fenwick was the son of Sir William Fenwick, who had been Sheriff of Northumberland in 1578 and 1589, and his first wife Grace Forster daughter of Sir John Forster of Edderstone and Hexham.He was knighted at Royston on 18 January 1605 and succeeded his father on 1618 at the age of 35.Fenwick himself also served as Sheriff in 1619–20 and 1644–45.