In 1746 he was commander of the sloop HMS Jamaica, which captured two French privateers while employed as a cruiser in the channel.

In 1759, during the Seven Years' War, he commanded the Portland, one of the ships employed under Commodore Robert Duff in the blockade of Quiberon Bay, and was present at the total defeat of the French on 20 November.

In December 1779, Arbuthnot conveyed the troops of Sir Henry Clinton to Charleston, South Carolina, and cooperated with him in laying siege to that city.

That he was ignorant of the discipline of his profession was proved by his altercation with Sir George Rodney; that he was destitute of even a rudimentary knowledge of naval tactics was shown by his absurd conduct of the Battle of Cape Henry; and, for the rest, he appears in contemporary stories (cf.

Morning Chronicle, 18 May 1781) as a coarse, blustering, foul-mouthed bully, and, in history, as a sample of the extremity to which the maladministration of Lord Sandwich had reduced the British Navy.