Captain Fox, had the temporary command of the Kent. During 1748 he was first lieutenant of the Invincible, guardship at Portsmouth, and for the next few years was on half pay in Scotland. In January 1755 he was appointed first lieutenant of the Prince with Captain Charles Saunders [q. v.], and on 22 April 1755 was promoted to command the Savage sloop, attached during the year to the western squadron cruising under the command of Sir Edward Hawke or Vice-admiral Byng.
On 23 March 1756 Lockhart was posted to the Tartar, a frigate of 28 guns and 180 men, in which during the next two years he was engaged in active, successful, and brilliant cruising in the Channel, capturing several large privateers of equal or superior force, among them the Cerf of 22 guns and 211 men, the Grand Gideon of 26 guns and 190 men, the Mont-Ozier of Rochelle of 20 guns and 170 men. In engaging the last, on 17 Feb. 1757, Lockhart was severely wounded, and obliged to remain on shore for the next two months. He had only just rejoined his ship when, on 15 April, off Dunnose, he captured the Duc d'Aiguillon of St. Malo, of 26 guns and 254 men; and on 2 Nov. the Melampe, of 36 guns and 320 men, a remarkably fine vessel, which was added to the navy as a 36-gun frigate. The admiralty acknowledged the brilliant service by a complimentary letter, and by promoting Lockhart to the command of the 50-gun ship Chatham; by promoting the Tartar's first lieutenant to the rank of commander, and desiring Lockhart to name one of the subordinate officers to be promoted to the vacancy. Lockhart replied that unfortunately none of the young gentlemen had more than four years' time, and recommended that the promotion should be given to the master, which was done. He was also presented by the merchants of London and of Bristol with handsome pieces of plate ‘for his signal service in supporting the trade;’ and by the corporation of Plymouth with the freedom of the borough in a gold box.
Lockhart's activity had severely tried his health, and he spent the next few months at Bath, waiting for the Chatham to be launched. This was done in April 1758, and, as a further mark of admiralty favour, the officers and most of the men of the Tartar were also appointed to the Chatham. By the middle of May she was ready for sea, and from June to September was in the North Sea, cruising in quest of the enemy's privateers, but without any marked success. In September she was ordered into the Channel, and through the following year formed part of the fleet under Sir Edward Hawke; she was, however, detached during the summer off Havre under Rear-admiral George Brydges (afterwards Lord) Rodney [q. v.] In October she again joined Hawke, and was sent with Commodore Duff to keep watch in Quiberon Bay, which the small squadron left on the morning of Nov. 20, on the news of the French fleet being at sea. In the forenoon they were chased by the French fleet, which was thus delayed, overtaken, and brought to action by Hawke. Four days later Hawke appointed Lockhart to the Royal George in the place of Captain John Campbell (1720?–1790) [q. v.], who was sent home with the despatches. In the end of January 1760 the Royal George came to Spithead, and a month later Lockhart was appointed to command the Bedford of 64 guns, forming part of the fleet under Hawke or Boscawen (1760–1).
By the death of his brother James in September 1760 Lockhart succeeded to the Ross estate of Balnagowan, the entail of which obliged him to take the name of Ross; this he formally did in the following spring, announcing the change to the admiralty on 31 March 1761. He was then at Lockhart Hall, where he seems to have passed the winter on leave, but afterwards rejoined the Bedford during the summer. In September he applied to be relieved from the command, and on the 27th was placed on half pay. In the previous June he had been elected member of parliament for the Lanark boroughs, but it does not appear that he took any active interest in parliamentary business. He devoted himself principally to the improvement of his estates and the condition of the peasantry, and became known as ‘the best farmer and the greatest planter in the country; his wheat and turnips showed the one, his plantation of a million of pines the other’ (Pennant, Tour through North Britain).
In 1777, when war with France appeared imminent, Ross returned to active service, and was appointed to the Shrewsbury, one of the fleet with Keppel in the action off Ushant on 27 July 1778. On 13 Aug., by the successive deaths of his elder brothers without male issue, he succeeded to the baronetcy. On 19 March 1779 he was promoted to the rank of rear-admiral, and during the summer, with his flag in the Royal George, he was fourth in command in the Channel. In September he was sent with a small squadron into the North Sea to look out for John Paul Jones [q. v.], but Jones, after capturing the Serapis in 1779, made good his escape. Continuing in the Channel fleet, Ross was
