Thomas Lemon was appointed 2nd Lieutenant, Royal Marines in 1798 and promoterd to 1st Lieutenant in 1803, serving aboard H.M.S. Dreadnought at the Battle of Trafalgar as one of her four Royal Marines officers.
H.M.S. Dreadnought, 98 guns, was the sister ship to the "Fighting Temeraire", built in Portsmouth Dockyard from the desighn of Sir John Henslow, Surveyor of the Navy. Launched on 13 June 1801, she was the first man-of-war launched after the union of Great Britain and Ireland. She bore the flag of Admiral Hon. William Cornwallis with the Channel Fleet, during the blockade of Brest in 1803 and the flag of Vice Admiral Cuthbert Collingwood with the blockading force before Cadiz in 1805, only to be superseded as Collingwood's flagship by the Royal Sovereign just 10 days before Trafalgar. Thomas Lemon would have served with the legendary man before he left H.M.S. Dreadnought, and the ship lost the title of second-in-command's flagship at that great battle by little over a week. Lining up towards the rear of Collingwood's column under the Captaincy of John Conn, the Dreadnought became heavily engaged with the Spanish ships, San Juan Nepomuceno and Principe de Asturias, and the French ship, Indomptable. San Juan Nepomuneco had already been severly damaged and Dreadnought ran on board in little more than quarter of an hour. The Principe de Asturias, to which she next devoted her attention, got away. Dreadnought's losses in the battle amounted to 33 killed and wounded. She had her masts cut with shot and her maintop sail-yard shot away.
Thomas Lemon was promoted to Captain in 1812 and Major on 22 July 1830, reaching the rank of Honorary Lieutenant-Colonel in 1854, dying in 1856.