MacCarthy was appointed in 1812 by the British as military governor of former French territories Senegal and Gorée, after Napoleon was defeated in Russia and retreated with high losses.
When the Napoleonic Wars ended, the United Kingdom returned these colonies to France in the Treaty of Paris in 1814, and MacCarthy was appointed governor of Sierra Leone.
Returning from Honduras on the transport HMS Calypso in June 1798 with the grenadier company of that regiment, MacCarthy was wounded whilst in a day-long action fighting off a French privateer.
He was appointed a major in the New Brunswick Fencible Infantry (later the 104th Foot) on 14 April 1804 and remained with them until 1811, when at the age of about 57, he received a lieutenant-colonelcy in the Royal African Corps.
This was the colony established by the British in West Africa in the late eighteenth century for the resettlement of Black Loyalists from North America and London after the Revolutionary War.
As governor, MacCarthy took a strong interest in the welfare of the colony, actively encouraging the building of housing and schools for the settlers.
In addition, he arranged for the support and education of native children whose parents had been captured by slavers, in schools run by the Church Missionary Society.
In 1818, MacCarthy signed a treaty with Mangé Demba, according to which the Îles de Los (off the coast of Guinea) were ceded to the British in exchange for an annual rent.
After the African Company of Merchants was abolished in 1821, for its failure to suppress the slave trade efficiently, Great Britain took on the Gold Coast as a crown colony.
The next day, at around 2pm, they encountered a large enemy force of around 10,000 men; in the belief that the Ashanti army contained several disaffected groups whose chiefs were willing to defect, MacCarthy instructed the band to play the British National Anthem loudly.
Only one additional barrel of powder and one of shot were brought up, and ammunition ran out around 4pm; the Ashanti made a determined attempt to cross the river, and quickly broke into the camp.