Duff was promoted to the command of the TERROR bomb vessel off the coast of Scotland in 1744 and then to the Anglesea between 1747 and 1748 off the coast of Ireland. In 1755 he was appointed to the ROCHESTER in the Channel. He contributed to the victory of Quiberon Bay, 1759, by leading the French in pursuit of his small squadron to bring them within range of the main British fleet. His next command was the FOUDROYANT in the West Indies where he was present at the reduction of Martinique in 1762. In 1775 he was promoted to rear-admiral and appointed Governor and Commander-in-Chief, Newfoundland. His last command was as Commander-in-Chief, Mediterranean, in the Panther between 1777 and 1780. He was promoted to vice-admiral in 1778.
The papers were presented by Lieutenant-Colonel Scott Duff of Fetteresso in 1947. A further section was bequeathed by Miss Gladys Duff in 1965. There are logs, 1744 to 1747, 1749 to 1762 and for part of 1779; letter and order books, 1745 to 1762, 1775 to 1780; a register of Newfoundland fishing vessels, 1775; a list of ships, 1770; various signals and sailing directions and a family account book, 1769 to 1778. An extensive collection of charts was also presented by Lieutenant-Colonel Duff in 1947 and is held by the Navigation Department.