Cornwallis remained in "Dunkirk" when she was assigned to the Mediterranean fleet then commanded by Admiral Charles Saunders. "Dunkirk" was detached on blockade duty, ensuring the French fleet remained in the city of Heraklion, Crete. Cornwallis moved to Saunders' flagship where he remained for little over a year. On 5 April 1761 Cornwallis passed his examination for lieutenant and was promoted into the newly commissioned third-rate . In July 1761 Cornwallis was with "Thunderer" and two other line-of-battle ships blockading Cadiz. Two French ships escaped the blockade and the British squadron set off in pursuit. "Thunderer" caught up with the 64-gun and captured her in a single-ship action that lasted about half an hour. The British lost seventeen killed and one hundred and thirteen wounded.