In 1777, when war with France appeared imminent, Ross returned to active service, and was appointed to the 74-gun , joining the fleet under Admiral Augustus Keppel in the Battle of Ushant on 27 July 1778. On 13 August, 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, but Jones, after capturing the "Serapis" in 1779, made good his escape. Continuing in the Channel Fleet, Ross was with Rodney at the capture of the Caracas convoy, the Battle of Cape St. Vincent and the relief of Gibraltar in January 1780; with George Darby at the relief of Gibraltar in April 1781; and with Lord Howe during the early summer of 1782. On the return of the fleet to Spithead in August he resigned his command, and had no further employment afloat. He became a vice-admiral on 24 September 1787, and died at Balnagown Castle in Ross-shire on 9 June 1790. He married Elizabeth, daughter of Robert Dundas of Arniston, the younger, in 1762 and had a number of children. Among them was his eldest son, Charles Lockhart-Ross, an army officer who inherited the baronetcy on his father's death, and George Ross, father of distinguished legal writer George Ross.