Fenwick, whose family had often represented Northumberland, was described by Lord Tankerville, its lord lieutenant, as a Jacobite, who ‘ought not to represent the county’.
After standing unsuccessfully for Northumberland in 1734, he was returned unopposed in 1741 and 1747. He voted against the Government on the chairman of the elections committee in 1741 and the Hanoverians in 1744, but was absent from the divisions on them in 1742 and 1746. At the general election of 1747 he was ‘in such a bad state of health’ that he was not expected to ‘last 12 months’.
