The fort was built on a rocky point of land near the mouth of the Thames River on Long Island Sound; it was completed in 1777 and named for Governor Trumbull, who served from 1769 to 1784.

[3][4][5] Benedict Arnold had betrayed his country by this time, and he was serving as a brigadier general in the British Army when he led a raid on Groton and New London, Connecticut, on September 6, 1781.

The British were finally able to enter the fort, and militia commander Lieutenant-Colonel William Ledyard offered his sword to Loyalist officer Major Bromfield as a token of surrender.

According to contemporaneous accounts, Bromfield took the sword from Ledyard and drove it through him, and Arnold's men proceeded to show no quarter to the remaining American troops.

Arnold had long promised to visit New London and the neighbouring towns on the sea shore, and the enemy having frequently appeared in sight of the harbour, the alarm guns fired on the present occasion were considered by the country around us the salutes of prizes or other vessels belonging to the port.

The action was then renewed, when the flag staff was unfortunately shot away; notwithstanding which the defence was gallantly continued until about five or six hundred of the enemy having forced the pickets had entered through the breach.

After massacring the living they insulted the dead, by actions too horrid to mention --- the bodies were arranged alongside of each other for the purpose, and, to shew contempt to Col. Ledyard, they singled out a Negroe to place next to him.

Before their departure, which was in the evening of the same day they landed, they burnt all the shipping that could not get up Norwich river, among which were several rich prizes lately arrived, with their cargoes, some of which were stored and the rest on board the vessels – about 40 sail, all on fire, were floating up and down the stream.

The prize brig Hope, laden with provisions, happily escaped the general conflagration, altho' the fire from the shore several times caught the awning which covered her quarter deck, and went out --- several vessels in full blaze passed within two and three feet of her; ten lay within forty yards, and consumed to the wateredge, but Providence directed she should escape, and a very fortunate one it was, as there was no other supply of provisions in town.

Secretary of War Henry Dearborn's report on fortifications for December 1811 describes the fort as "an irregular enclosed work of masonry and sod, mounting 18 heavy guns [with] a brick barracks for one company".

[9] The new fort was built under the supervision of Army engineer George Washington Cullum, who later served as superintendent of the United States Military Academy at West Point, New York.

[4] One of the Merchant Marine officers trained there was actor Jack Lord, who is best known for playing Steve McGarrett on the popular TV series Hawaii Five-O in the 1960s and 70s.

[12] A result of this work was that Fort Trumbull was the location for the Naval Underwater Sound Laboratory from 1946 to 1970, which developed sonar and related systems for US Navy submarines.