The Biographical History uses extensively the entry on John Pascoe Grenfell in the Dictionary of National Biography and Brian Vales' A war betwixt Englishmen: Brazil Against Argentina on the River Plate and Independence or death: British sailors and the Brazilian independence, 1822-1825, see the bibliography for further details.
John Pascoe Grenfell was born at Battersea on 20 September 1800, the son of J.G.Grenfell and probably nephew of Pascoe Grenfell. At the age of eleven he entered the service of the East India Company and after making several voyages to India he joined the service of the Chilean Republic Navy under Lord Cochrane in June 1820 . Grenfell was made a junior lieutenant and took part in most of Cochrane's exploits in the war of Chilean independence most notably in the cutting out of the Esmeralda, when he was wounded. The war now over the Chilean Navy was demobilised, Cochrane was offered the command of the Brazilian Navy. In 1823 Grenfell accompanied Cochrane to Brazil, with the rank of commander, and served under his orders in the war with Portugal specially distinguishing himself in the evacuation of the Para. Commanding the 18 gun brig-of-war the Maranhãeo Grenfell was ordered to secure the adherence of Para to the Brazilian Empire. He managed to keep an uneasy peace in Para and secured the province for the Empire and seized the frigate Imperatriz, he was later arrested for his actions in Para but was released without charge. Grenfell commanded the brig Caboclo in the River Plate in May 1825 in the build up to war with Argentina. In December 1825 Brazil declared war on Argentina, Grenfell witnessed active service in the war with Argentina. After one battle outside of Montevideo he was mentioned favourably in reports by Vice Admiral Lobo, "The valour which Captain Norton, and Commander Grenfell of the Caboclo ... showed in fighting the enemy for so long; and the promptitude with which they followed did them honour"( Vale, B., 2000, p.52). Afterwards under Commodore Norton, in the action off Buenos Ayres at Lara-Quilmes on 29 July 1826 he lost his right arm whilst engaging the Argentine warship the 29 de Mayo.
After the loss of his arm, Grenfell went to England to 're-establish' his health, but returned to Brazil in 1828 to Captain the Brazilian Navy corvette Maria Isabela. The war was drawing to a close but Grenfell still managed to capture the privateer Peruana. Grenfell had married in Montevideo in 1829 to Dona Maria Dolores Masini, he had several children including Harry Tremenheere Grenfell and an elder son John Granville Grenfell. After the war although he spent four years on leave from 1830-1834 , Grenfell continued his service with the Brazilian Navy. In 1836-1837 he commanded the squadron on the lakes of the province of Rio Grande do Sul against the rebel flotillas which he managed to capture or destroy, compelling the rebel army to surrender. As a reward for his success Grenfell was promoted to Commodore however the Brazilians then failed to capitlize on Grenfell's success and use the army to secure the region. Within two years the rebels had regained significant strength under the leadership of Italian revolutionary Giuseppe Garibaldi. The Brazilians again looked to Grenfell to put down the rebel forces in Rio Grande do Sul, in May 1839 he was reappointed in the Rio Grande do Sul and quickly defeated the the rebel forces restoring control of the region to the Brazilian government. Grenfell was promoted to Rear Admiral in 1841 and was put in charge of the naval stations of southern Brazil during the 1840s.
In 1846 Grenfell was appointed consul-general in England and lived in Liverpool and in August 1848 while superintending the trial of the Dom Affonso, a ship of war built at Liverpool for the Brazilian government, he assisted in saving the lives of the passengers and crew of the emigrant ship Ocean Monarch, burnt off the mouth of the Mersey. Grenfell's actions resulted in his receiving the thanks of the Corporation and the gold medal of the Liverpool Shipwreck Society. In 1851 war broke out between Brazil and the Argentine Republic, Grenfell returned to take command of the Brazilian navy and achieved a famous victory at Tonelero. After the peace he was promoted to Vice Admiral, and in December 1862 to Admiral; in 1852 he returned to Liverpool, and resumed his function as consul-general, holding the office until his death on 20 March 1869 . John Pascoe Grenfell was one of many British men to serve in the Brazilian navy during the 19th century, despite this Grenfell's achievements in South America mark him out as a remarkable man "From the beginning to the end, his career was spectacular and his abilities and personality were admired by all who met him"( Vale, B., 2000, p.236)