[2] Following the example of his uncle, Admiral Charles Cornewall, he embarked on a naval career, being promoted to the rank of Lieutenant on 13 March 1734.

Outgunned by the enemy, she came under a heavy bombardment during which James Cornewall was mortally wounded when a Chain-shot carried away both his legs.

Given command of the newly built HMS Colchester in 1744, he was sailing the ship under the guidance of a pilot out of the Nore anchorage and on to the Downs on 21 October when it ran aground between Long Sand and the Kentish Knock.

[3] In June 1749 he made a generous donation to Thomas Coram's Foundling Hospital, and encouraged the officers and crew of the Sunderland to raise money for the same cause.

It was Cornewall's evidence to the court-martial that followed that was largely responsible for Admiral John Byng being convicted and executed.