He later saw action during the blockade of Brest during the Napoleonic Wars before becoming commanding officer of a sloop which was sent to the Aegean Sea to defend the population of Malta from pirates; the grateful people presented him with a sword.

He then sailed to the Adriatic Sea where he led a naval brigade providing artillery support to the Austrian forces during the siege of Trieste.

He went on to be senior naval officer at the Cape of Good Hope and then senior officer at Mauritius, with orders to suppress the slave trade: he concluded the Moresby Treaty with Seyyid Said, the imam of Muscat, restricting the scope of local slave trading and conferring on English warships the right of searching and seizing local vessels.

[1] He was assigned to the second-rate HMS London as an able seaman but was promoted to midshipman and took part in the unsuccessful expedition to capture Ferrol in Spain in August 1800 during the French Revolutionary Wars.

[2] In HMS Amazon he took part in the pursuit of the French Fleet, under the command of Admiral Pierre-Charles Villeneuve, to the West Indies and back in Summer 1805.

[4] He then returned to the Adriatic Sea where he led a naval brigade providing artillery support to the Austrian forces during the siege of Trieste in October 1813.

[6] Promoted to rear admiral 20 December 1849,[7] Moresby became Commander-in-Chief of the Pacific Station, with his flag in the fourth-rate HMS Portland in August 1850.