As Lieutenant Colonel of the 80th Foot he saw action in the American Revolutionary War,[1] serving under Benedict Arnold in the raid against Richmond. Under Arnold and William Phillips, he was present at the capture of Williamsburg, Blandford, the attack on Osborne's, and Manchester. Then he passed under the command of Charles Cornwallis.[1] He commanded the left wing at Green Spring. With Banastre Tarleton, he was bottled up by the Marquis de Choisy at Gloucester during the siege of Yorktown. Assigned as joint commissioner for carrying out the capitulation,[1] he was made a prisoner of war after the fall of Yorktown. He was promoted to Colonel on 20 November 1782. After repatriation, he was appointed to the board of commission in 1782 to examine claims for compensation to those "who having remained loyal to the mother country, had suffered in their rights, properties and profession."
Dundas married Lady Elizabeth Eleanora (d. 1837), daughter of Alexander Home, 9th Earl of Home, on 9 January 1784.[1] He was elected MP for Orkney again in 1784 and sat until 1790.[2]
When the French later regained possession of the island Victor Hugues issued a declaration on 10 December, which stated "That the body of Thomas Dundas, interred in Guadeloupe, shall be taken up and given as prey to the birds of the air."[1] This aroused great outrage in England and prompted a memorial in St Paul's Cathedral.[1][3] This was erected in 1805 and was sculpted by John Bacon.[4]