His half-brother, Granville Leveson-Gower, inherited his father's earldom and would use his political influence to help John's career.

His first command was the fireship HMS Salamander, in which he saw action at the Battle of Lagos on 18 August 1759, serving under Admiral Edward Boscawen.

Leveson-Gower was advanced to the rank of post captain on 30 June 1760 and took the 32-gun HMS Quebec to the Mediterranean to serve under Sir Charles Saunders.

He was present at the Battle of Ushant on 17 July 1778, where he strongly supported Admiral Augustus Keppel, Valiant sustaining casualties of six killed and twenty-six wounded.

[1] Leveson-Gower returned to service after the fall of the North Ministry in March 1782 and was appointed first captain of HMS Victory under Lord Howe, and served in that capacity both in the Channel, and later on at the relief of Gibraltar and the skirmish off Cape Spartel.

[8] They had several sons, including John, who became a general and MP, Edward, who became a rear-admiral, and Augustus, who was a captain and drowned aged 21.