Downman spent most of the American War of Independence as a midshipman, with a spell in French captivity after his ship was wrecked off the coast of Ushant while chasing an enemy frigate.

Downman went on to command several ships of the line, often as a flag-captain, and took part in the capture of the Cape Colony and operations on the Río de la Plata.

Left without active employment after the end of the wars with France, he briefly returned to service in 1824. before being promoted to flag rank the following year.

His uncle was Dr Hugh Downman, a physician and poet, who obtained for his nephew an offer to serve aboard the 32-gun HMS Thetis as a midshipman.

[3][4] Edgar at this time was flying the broad pennant of Commodore William Hotham, and in October that year was part of the fleet sent out under Lord Howe to relieve Gibraltar.

[3] Little is known of Downman's activities during the years of peace that followed, until his appearance in February 1789 with the fleet despatched to the East Indies under Commodore William Cornwallis.

With the commencement of the French Revolutionary Wars, Downman joined the 74-gun HMS Alcide and went out to the Mediterranean, where served with Commodore Robert Linzee's squadron.

[5] The two ships fought each other for two days, and by the second Downman had exhausted his supply of shot, and resorted to firing nails and pieces of iron hoop at his opponent.

[5] As a reward for his good service Downman was advanced to post-captain on 26 December 1798 and was appointed to command the 32-gun HMS Santa Dorothea, a frigate that had recently been captured from the Spanish.

He was supported the operations under Lieutenant-General Sir David Baird to capture the Cape Colony, and landed in Table Bay at the head of a party of marines and two howitzers.

[5] He was sent home with the despatches announcing the capture of the colony, after which he was sent out to the Rio de la Plata, where he resumed command of his old ship, Diomede.

[5] Downman's next appointment was to superintend the prison ships moored at Portsmouth, a task he carried out until January 1811, when he was given another seagoing command, the 74-gun HMS Princess Carolina and attached to the fleet in the North Sea.

[5][9] He took part in the destruction of the 40-gun French frigate Amazone off Cape Barfleur, conveyed a fleet of East Indiamen to Madeira and carried out cruises in the North Sea.

[5] Politically Downman was an ardent supporter of Whig Charles James Fox and opposed the Melvilles, Henry and his son Robert, who were for a number of years powerful at the Admiralty.

[5] He was described as a "strict disciplinarian, but an accomplished gentleman; well-read...[and] temperate..."[5] Whilst serving in the Mediterranean he was apparently especially disliked by Emma Hamilton, with Downman's obituary noting that "from this fact some slight judgement of his character may be formed.