He served on a number of ships during the War of the Austrian Succession, seeing action at both the First and Second Battles of Cape Finisterre, having by then risen to the rank of lieutenant.

Taking the additional name Ross after he inherited a deceased relative's estates, he served as a member of parliament and undertook land reforms and improvements during the years of peace before the outbreak of the American War of Independence.

He served in several actions as a junior commander of Rodney's fleet, including the capture of the Caracas convoy, the Battle of Cape St. Vincent and the relief of Gibraltar.

He was then appointed to his first command, that of the fireship HMS Vulcan, in which he saw action with Sir Edward Hawke's fleet at the Second Battle of Cape Finisterre on 16 October 1747.

[1] As a reward for Lockhart's services, the Admiralty offered him the command of the 50-gun HMS Chatham, which was then nearing completion, and making several promotions from Tartar's crew.

He was also presented by the merchants of London and of Bristol with 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.

[1] She spent the summer under Rear-Admiral George Brydges Rodney, taking part in the Raid on Le Havre before rejoining Hawke in October, and then being sent to join a squadron under Commodore Robert Duff, to watch the French in Quiberon Bay.

[1] Four days later Hawke appointed Lockhart to command HMS Royal George in the place of Captain John Campbell, 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 64-gun HMS Bedford, forming part of the fleet under Hawke or Edward Boscawen.

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'.

[4] In 1777, when war with France appeared imminent, Ross returned to active service, and was appointed to the 74-gun HMS Shrewsbury, joining the fleet under Admiral Augustus Keppel in the Battle of Ushant on 27 July 1778.