Cornewall inherited his father’s share in the banking firm Amyand (subsequently Cornewall), Staples, and Mercer, and seems to have been associated with it until 1776.
At the general election of 1774 he successfully contested Herefordshire (which his father-in-law had represented 1722-68). His first recorded vote was against Administration on Wilkes, 22 Feb. 1775, and he again voted in opposition on the civil list debts, 16 Apr. 1777. No further vote of his is recorded till 12 Feb. 1779, when he voted with Opposition on the contractors bill; after this he voted regularly against Administration until the fall of North. He supported Shelburne’s peace preliminaries, 18 Feb. 1783, but did not vote on Fox’s East India bill, 27 Nov. 1783. He was at the St. Alban’s Tavern meeting of country gentlemen in January 1784 which advocated the union of parties.
In William Adam’s list of the new Parliament Cornewall was classed as Opposition, and he voted against Pitt on the Regency. Two or three short speeches of Cornewall’s on different subjects are recorded.
He died 26 Sept. 1819.