"Monarch" had a very active career, fighting in her first battle in 1778 at the First Battle of Ushant and her second under Admiral Rodney at Cape St. Vincent in 1780. She fought in the van of Graves' fleet at the Battle of the Chesapeake in 1781 under Admiral Francis Reynolds. In early 1782 she was actively engaged at the Capture of Sint Eustatius, Action of 4 February 1781, the Battle of Saint Kitts, the Battle of the Saintes and, the Battle of the Mona Passage. In 1795 she was deployed as part of the small fleet under Admiral George Elphinstone that captured the Cape of Good Hope from the Dutch East India Company at the Battle of Muizenberg. In 1797 "Monarch" was Vice Admiral Richard Onslow's flagship at the Battle of Camperdown, under Captain Edward O'Bryen, and in 1801 she was part of Admiral Nelson's fleet at the Battle of Copenhagen, where her captain, James Robert Mosse was killed and she suffered over 200 casualties including 55 dead, the highest number of casualties of any ship engaged in the battle.