[1] After the outbreak of the American Revolutionary War, he transferred to HMS Foudroyant in 1775 under Captain John Jervis (later Earl St Vincent), who two months later rated him Midshipman.

[1] During this time little is known of his life, as he left no journals or records and atypically of a young naval officer did not make a stir in society.

Soon after taking command he was involved in the capture of the French frigate Franchise in the English Channel and briefly served as Admiral Collingwood's flag-captain in the same ship.

[1] In the spring of 1807 he was flag captain to Rear Admiral William Essington, third-in-command at the bombardment of Copenhagen, where he benefited from the prize money for the captured ships in the harbour, but was forced to retire from active service through ill-health on his return at the end of the year.

[3] Although a man in his position could have been expected to have received promotion and rewards at the end of the Napoleonic Wars, Mansfield never survived long enough, dying at Rochester after a period of illness in 1813.

For many years he commanded the Minotaur, and had the Honor of capturing the El Neptuno a Spanish 80 Gun Ship in the ever memorable engagement of TRAFALGAR Soon after this Period, worn down by fatigue he retired into the bosom of his Family, and, after a long and afflicting illness, calmly resigned his Soul into the hands of his Maker