He entered the Navy in 1767 as a midshipman on board the , commanded by his uncle, and family interests gained him rapid promotion. He was commissioned as a Lieutenant on 12 February 1777, was promoted to Master & Commander on 9 March 1778, and was made a Post Captain on 9 October 1778, at the age of only 22, and appointed to the 32-gun frigate HMS Raleigh.
In 1783, at the end of the American War of Independence, he was placed on half-pay, but when the French Revolution broke out in 1789 he was appointed to command the 74-gun under Lord Howe. As captain of the Defence Gambier saw action at the battle of the Glorious First of June in 1794, gaining the distinction of being the first ship the break through the enemy line. He subsequently received the Naval Gold Medal and was appointed Colonel of Marines (a sinecure involving no duties, but excellent pay).
In 1807, he was still a Lord of the Admiralty, but also saw action in the 2nd Battle of Copenhagen, where with General Lord Cathcart, he captured the Danish Navy, including stores on land, for which he received official thanks from Parliament, and on 3 November 1807 a peerage, becoming Baron Gambier of Iver.
In 1809, he threw away the destruction of the French Fleet in the Battle of the Basque Roads in one of the Royal Navy's biggest embarrassments - but also one of its most cherished victories.
A furious Cochrane accused his fleet commander of cowardice in the face of the enemy and was not alone in his criticism. Admiral Sir Eliab Harvey, who had commanded the famous "Fighting Temeraire" at the Battle of Trafalgar, was one of the seagoing officers of the fleet who were bitter at the missed opportunity. He told Gambier to his face: "I never saw a man so unfit for the command of a fleet as Your Lordship."
Several factors may have contributed to Gambier's decision. A deeply religious man, he had been opposed to attacks by fireships from the beginning for ethical reasons and may have been dissuaded even further by the fact that the proposal came from Cochrane, who was known for his extremely destructive methods and his disregard for collateral damage. The use of fireships in itself was not without its peril even to the attackers and it should be mentioned that in fact many of them failed utterly to achieve their purpose of destroying French ships. In fact, the damage to the French fleet may have resulted more from their own desperate (and largely unnecessary!) manoeuvring to avoid the fireships than from the fireships themselves.
Cochrane's accusations were severe enough that Gambier demanded, and got, a court-martial for himself, to clear his name. Both Lords Gambier and Cochrane were members of Parliament and Cochrane should have been wise to the fact that it would turn into a political farce. An old friend of Gambier's was appointed to chair the court-martial and Gambier's "old-boy" stature was enough that he was acquitted.
Modern scholars typically now acknowledge that Gambier prevented Cochrane from achieving a critically important naval victory, and also point out that while Gambier possessed principle and courage, his administration of the Admiralty was also one of the most scandal-ridden known.
Gambier was a founding benefactor of Kenyon College in the United States, so the town that was founded with it, Gambier, Ohio is named after him. Mount Gambier, South Australia, the extinct volcano and the later city, are also named after him.