Resuming his command in the North Sea after the battle, Stewart resigned his position towards the start of 1782 when he failed to stop a Dutch convoy escaping him in the Downs.
[2][4] In Lynx he served in the Downs, and in August the ship formed part of the naval escort that brought Charlotte of Mecklenburg-Strelitz to England for her marriage to George III, arriving at Harwich on 6 September.
[3] This situation changed with the start of the American Revolutionary War, and he was appointed to command the brand new 74-gun ship of the line HMS Berwick on 27 December 1777.
[11][12][13] While Berwick was still at sea disagreements between the two commanders of the British at Ushant, Admiral Augustus Keppel and Vice-Admiral Sir Hugh Palliser, had led to political controversy.
[3] The First Lord of the Admiralty at the time, John Montagu, 4th Earl of Sandwich, specifically noted that: Captain Stewart behaved exceedingly well in the action, is a very good officer and has kept himself clear of disputes of every kind, and shown no particular attachments.
[3] With his ship having received a refit in December, Stewart was sent with Berwick to the West Indies on 8 April 1780, in a force commanded by Commodore Robert Boyle-Walsingham.
[11][12] Berwick was repaired in a refit that was completed in February 1781, and Stewart was subsequently appointed a commodore and Commander-in-Chief, North Sea, on 27 March.
[11][15] Afterwards Parker resigned his command in the North Sea and Brathwaite also left the station, leaving Stewart able to re-hoist his broad pennant as commodore in September.
After this it fought the Battle of Cape Spartel on 20 October, with Cambridge serving as one of the seconds to Vice-Admiral Mark Milbanke's flagship the 90-gun ship of the line HMS Ocean.
[16] Cambridge was sent in for a refit prior to a planned sailing to join the West Indies Station, but this was still in progress when the American Revolutionary War came to an end in 1783, at which time the posting was cancelled and the ship paid off.
On 19 February 1762 Stewart replaced Montgomerie as member of parliament (MP) for Wigtown Burghs, but in March he gave up the seat as part of compromises being made by his father over the family's political control.
Wanting to truly take up the position, at the 1784 British general election he gave up his seat to his opponent, Andrew McDouall, with the understanding that Stewart could return to Wigtownshire if he ever lost the receivership.
[3] With this completed in July, Stewart appointed a deputy to work as receiver general and instead focused on his estates and various coal and iron mines, being both an agricultural improver and mineral developer.