While in command of HMS Monmouth he was caught in the Nore Mutiny of 1797 and was the officer selected to relay the demands of the mutineers to George III.

[2][5][6] Carnegie served on Apollo for just under two years before joining the flagship of Rear-Admiral Sir John Lockhart-Ross, the ship of the line HMS Royal George.

[1] In early 1780 he joined the newly recommissioned HMS Sandwich, flagship of Rodney, sailing for the Leeward Islands Station.

[2][8] While the battle itself was inconclusive, Sandwich fought alone against de Guichen's flagship Couronne and two of her consorts for an hour and a half, taking a great amount of damage.

[Note 2][2][10][11] On 7 April 1782 Carnegie was promoted to post captain and given command of the frigate HMS Enterprise which had newly arrived on station in the Leeward Islands from England.

His elder brother David died in 1788 leaving him his father's heir and holding the courtesy title of Lord Rosehill.

[18][2] The Peace expired in May 1803 and Carnegie was given the 100-gun first-rate HMS Britannia in June to again serve in the Channel Fleet of Admiral William Cornwallis at the blockade of Brest.

[2] Britannia was a slow ship that did not sail well, and so Nelson ordered Carnegie to 'assume a station as most convenient' during the attack, allowing him the best chance to reach the battle on time.

[2][19] After the battle was won the British began to secure their prizes, but a large storm meant that many of the newly captured ships had to be abandoned; Carnegie ignored Collingwood's orders to leave the prisoners of war on board the ship nearest to him, Intrépide, and had Britannia's boats rescue them all before scuttling the prize.

The company held an important history in the economic development of Scotland, as it stimulated industrial investment in the production of linen and spinning factories across the rural Highlands and the East Coast.

[31] In 1796 Carnegie, as Earl of Northesk, was elected to serve as one of the sixteen Scottish representatives in the Parliament of Great Britain.

[32] Carnegie died on 28 May 1831 in Albemarle Street, London after a short illness, and was buried alongside Nelson and Collingwood in the crypt at St Paul's Cathedral, where his tomb and memorial slab can still be seen.