He spent most of his service in South America campaigns, most notably under the leadership of Lord Cochrane and Commodore Norton. Early history In 1811 he entered the service of the British East India Company, and in 1819 he joined the Chilean republic under Lord Cochrane. He took part in most of the conflicts undertaken by Lord Cochrane during the War of Chilean Independence, and rose to the rank of lieutenant.
On 5 November 1820 Grenfell was injured in the cutting of the Esmeralda the last major Spanish warship in the South Pacific.
In 1823 now holding the rank of commander he followed Lord Cochrane to Brazil, to fight in the Brazilian War of Independence. In August 1823, Grenfell in command of the small brig Dom Miguel sailed to Pará and using a similar tactic used by Lord Cochrane at Maranham, forced the surrender of the junta governativa by making them think a larger fleet was following.
On 29 July 1826, under the command of Commodore Norton, he lost his arm in the Cisplatine War. He then returned to England to recuperate.
Later life In 1828 Grenfell returned to Brazil, and during 1835-1836 he commanded a squadron of ships on the lakes of Rio Grande do Sul against rebel flotillas.
He was successful in defeating his opponents which forced the surrender of the rebel army. In 1841 he was promoted to rear-admiral and in 1846 was consul general in England and he returned to Britain to live in Liverpool. In 1848 when supervising the Brazilian warship Dom Afonso he was involved in the rescuing of passengers from the Ocean Monarch off the coast of England.
In 1851 war broke out between Brazil and Argentina and he was posted to Brazil to take control of their Navy.
In December of that year he successfully forced the passage of the Paraná River. After the conflict ended he was promoted to vice-admiral and then finally admiral.
He again returned to Liverpool to resume his role as consul-general and remained there until his death in 1869. Family Grenfell married Maria Dolores Masini.
They had eight children.
John Granville (1829–1866), Maria Dolores (1831–?), Sophia (1833–1898), Maria Emma(1833–?), Alfred (1839–?), Flora (1841–1874), Harry Tremenheere (1845–1906) and Thomas Cochrane (1847-1849). Non-titled nobility honors Military honors.