Garnier was commissioned lieutenant on 28 September 1757.
He was promoted commander of the Lynx 10 on 1 February 1763, which vessel he retained until she was paid off in August of the following year. Continuing in employment, he joined the Hound 14 that autumn and took her out to Africa in the following January. During the summer of 1765 he briefly commanded the Shannon 28 on the African station after Captain Philip Boteler returned home, and he then rejoined the Hound before coming home to pay her off in September 1766. He immediately recommissioned his first command, the Lynx 10, which he took out to the Leeward Islands at the end of the year, and he retained her until posted captain on 3 August 1768.
From the end of 1768 he had the Scarborough 20 in the Leeward Islands, transferring on the same station to the Kennington 20 at the beginning of 1772 which he brought home in the following year. He next commanded the Hind 24 from August 1774 which he recommissioned before sailing to the Leeward Islands in October, and after also seeing service in North America he transferred to the Argo 28 in 1775, which vessel he brought home to Portsmouth to pay off in the following year.
In October 1776 Garnier joined the Southampton 32, going out to Jamaica from Cork in the following year with a convoy and arriving in June. He spent the latter part of 1777 cruising successfully on that station, but in November had cause to remonstrate with the captain of a French frigate which had interfered in his pursuit of a rebel brig.
Garnier returned to England in 1778 and commanded the America 64 briefly for Lord Longford in the winter before rejoining the Southampton and serving in the Channel fleet retreat of August 1779. He then sailed with Admiral Sir George Rodney’s fleet for the relief of Gibraltar at the end of the year, and was with Captain John Moutray’s convoy when it was attacked by the allies with a massive loss of treasure on 9 August 1780.
In 1781 he went out to Jamaica where he exchanged with Captain William Affleck of the Grafton 74, and he brought that ship home to pay her off in September.
Garnier did not see any further service and he died on 20 March 1787.