Thomas was born in 1729 to Jean Dumaresq and Ann Bokenham (niece of Captain William Bokenham),[2] and was a member of the notable Dumaresq family of the Channel Islands.
He would obtain a commission as a lieutenant in 1755 and in 1763 he would obtain his first command, of the British Cutter, named the "Esther".[3]
Thomas Dumaresq was promoted to Rear-Admiral in 1794, and a Vice-Admiral in 1795. On 1 January 1801, he was promoted to Admiral of the Blue.[4]
On 3 June 1777 Dumaresq was found guilty of impressment by the Court of Common Pleas of Antigua and had to pay £950 what is approximately £160,000 in today's currency. To Dumaresq's luck, Lord Sandwich brought forth the government's plan to deal with the issue of illegal privateering altogether. Privateer Commissions would be granted by the colonial governors to all current illegal privateers and them would be able to keep their prizes they have already taken. The only condition was that all charges against all naval officers were dropped. If Dumaresq's charges were not dropped it could have had distress effects for the navy that could result in a reluctance of naval officers to sign-on new seamen due to fear of being charged with of impressment.[7]
During 1780, Rear-Admiral Joshua Rowley was sent to reinforce Jamaica against a possible Spanish threat. However, in October 1780 a hurricane formed off the coast of South America and would damage many British vessels, with the transport ship Monarch being sunk. Dumaresq, who was the Captain of the Ulysses was caught in this storm on October 6, 1780, resulting in only the foremast standing on the Ulysses. Having to drop all guns on the upper deck, Dumaresq would not get to Jamaica until the 26th of October 1780.[11]
Charlotte Dumaresq who married her first cousin William Dumaresq, son of Captain William Dumaresq (brother of Thomas Dumaresq). They had the following issue;