Her first commander, Captain Thomas Collingwood, commissioned her for service in January 1778, and after fitting out she sailed for the Leeward Islands in June 1778 with the squadron under Vice-Admiral John Byron. She came under the command of Captain Robert Fanshawe in 1779, and under him saw action at the Battle of Grenada on 6 July 1779. "Monmouth" was heavily involved in the fighting with the comte d'Estaing's fleet, and was ordered to Antigua to carry out repairs. She returned to Britain at the end of the year and was refitted and coppered at Portsmouth between December 1779 and December 1780. She recommissioned in late 1780 under the command of Captain James Alms, and was immediately assigned to the squadron under Commodore George Johnstone.