Onslow's last promotion, to lieutenant general, occurred on 10 October 1747. On 15 February 1752, he was appointed Governor of Fort William, and in 1759, was removed to become Governor of Plymouth and military commander of the Western District. In early 1760, he was called upon to preside over the court-martial of Lord George Sackville for his conduct at the Battle of Minden, and concurrently of Lord Charles Hay for remarks made against Lord Loudoun, but died of a stroke on 16 March 1760 while presiding over the latter trial.