While in America, Mansfield was made lieutenant on 25 November 1778, when only 18, two years earlier than the official minimum age of 20, joining the ship of the line HMS "Albion" as a junior lieutenant. In March the following year he transferred to the 74-gun ship HMS "Sultan" in which he was in action at the Battle of Grenada the following year and the Battle of Martinique a few months later. At the start of 1780, Mansfield was made first lieutenant of the newly captured frigate HMS "Fortunée" which was present as a repeating frigate at the disastrous Battle of the Chesapeake which led to Cornwallis surrendering his army. In January 1782 Mansfield was again in action at the Battle of Frigate Bay during Hood's unsuccessful attempt to relieve Saint Kitts, and where Mansfield successfully made it through enemy lines under the cover of darkness to gain intelligence from the besieged British fort on Brimstone Hill.