[1] Fenwick was the son of Sir William Fenwick,[2] who had been Sheriff of Northumberland in 1578 and 1589,[citation needed] 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 in 1618 at the age of 35.
[4] Fenwick himself was temporarily disabled from sitting in January 1644[2] but was re-admitted to Parliament in June 1646.
[2] Fenwick owned substantial estates in Northumberland and Durham but financial difficulties caused him to sell the greater part of his holdings in 1650 to Sir William Blackett for £20,000.
[5] He had married, firstly, Catherine Slingsby, daughter of Sir Henry Slingsby of Scriven, Yorkshire and Frances Vavasour, and had a son John and two daughters.