[2] In 1811 he entered the service of the British East India Company, and in 1819 he joined the Chilean Navy under Lord Cochrane.

In August 1823, Grenfell now a Commander in the small brig Dom Miguel sailed to Belem do Pará and using a similar tactic used by Lord Cochrane at Maranham, persuaded the Portuguese forces to surrender by making them think a larger fleet was in the offing.

In retaliation, Grenfell had five random imprisoned soldiers shot in the Largo do Palácio, a broad public square, on October 17, 1823.

[5] During the war with Argentina in 1826, he commanded the brig Caboclo as part of the squadron blockading Buenos Aires under (British) Commodore James Norton.

On 29 July, while engaged in a naval battle against the Argentine fleet, whose commander was admiral William Brown, he lost his right arm.

[7] In 1848, when Dom Afonso was undergoing steam trials on the Mersey with Grenfell, the diplomatic corps and members of the Brazilian and French Royal families on board, the frigate rescued 160 passengers from the America bound immigrant ship Ocean Monarch which had caught fire one day out from Liverpool.

[8] The Brazilian government was so pleased with the performance of Dom Afonso that Grenfell was ordered to sign a contract for a second steam frigate called Amazonas.