Rear-Admiral Leveson-Gower (pronounced Lewison-Gore) was the second son of John, 1st Earl Gower, and his third wife, Mary, the daughter and co-heiress of Thomas Tufton, Earl of Thanet and widow of Anthony Grey, Earl of Harold. His father died when he was fourteen and his mother retired to her estate at Bill Hill in Hurst, where John spent his late teens, when not away being privately educated.

Leveson-Gower entered the Royal Navy and was commissioned as a lieutenant in 1758, during the Seven Years' War. He commanded the fireship Salamander at the Battle of Lagos and, from the early months of 1760, he was commander of the Kingfisher sloop. On 30th June, he was promoted to be captain of the Flamborough frigate. From her, he was quickly moved into the Quebec, which he commanded in the Mediterranean, till the peace. He afterwards commanded the Africa on the coast of Guinea and the West Indies; Aeolus frigate in the Mediterranean in 1766-7; the Pearl on the home and the Newfoundland stations in 1769-72; and the Albion guardship in Plymouth in 1774.

In 1777, he was appointed to the Valiant for service in the English Channel and, after the French joined the American Revolutionary War, he was one of the Admiral's seconds - the other being Captain Jeryis in the Foudroyant - in the 1st Battle of Ushant on 17th July 1778. Its unsatisfactory outcome led to the subsequent court martial of Adm. Augustus Keppel (later Viscount Keppel), the Commander of the Channel Fleet and Leveson-Gower's evidence was strongly in his favour. Upon Keppel's striking his flag after his aquittal, Leveson-Gower resigned his command and did not serve again until after the change of ministry in March 1782, when he was appointed 1st captain of the Victory with Lord Howe, and served in that capacity, both in the Channel and later on at the relief of the Great Siege of Gibraltar and the Battle of Cape Spartel.

From January to April 1783, and again from December 1783 to July 1788, Leveson-Gower was one of the junior lords of the admiralty with Lord Howe, continuing at the admiralty with the Earl of Chatham until January 1790. During this time, he hoisted a broad pennant in the Hebe frigate in 1785, for a summer cruise around Great Britain with Prince William, Duke of Gloucester; and in the Edgar in 1787, in command of the Channel Squadron. On 24th September 1787, he was advanced to be rear-admiral, and in the following summer hoisted his flag again in the Edgar in the Channel. During the Spanish armament in 1790, he was again first captain to Lord Howe.

Adm. Hon. John Leveson-Gower had married, in 1773, Frances, daughter of Adm. the Hon. Edward Boscawen, by whom he had four sons and two daughters. In 1785, his elderly mother died at Bill Hill after her clothes accidentally caught fire. His elder half-brother, the 2nd Earl Gower, tried to claim her Berkshire estates, but the courts ruled that they had never belonged to the first Earl and were purchased with the Countess' own money in her widowhood. So the admiral inherited Bill Hill after all. Two years later, he purchased a secondary estate centred on Barkham House, four-and-a-half miles away on the other side of Wokingham. The Admiral died of an apoplectic fit on 15th August 1792. He was buried in the family vault under Barkham Church and his eldest son, Lieut-Gen. John Leveson-Gower, moved into Bill Hill, while Barkham House became a dower house for his widow.