In 1756, Thomas being in his non age, was appointed joint guardians, Thomas Barnum and Eliphalet Barnum of Danbury.
Thomas Barnum served throughout the Revolutionary War, as a soldier and sergeant with Colonel Van Schoonhoeven's 12th New York Regiment. He was at the Battle of Trenton, and is said to have been taken prisoner by Indian allies of the British at Ballston, New York, October 17, 1780, and kept prisoner at Montreal.
Thomas removed from Danbury, CT to Ballston, NY in the winter of 1774 and there bought a farm and built a log house. From April 1775 until the surrender of General Burgoyne he was a member of the militia company commanded by Captain Tyrannus Collins, in Colonel James Gordon's Regiment, and was frequently called out with that company to render short tours of duty whenever the occasion demanded. In that manner, during the period mentioned above, he was in actual service as a sergeant during at least twelve months. On October 17, 1780 he was taken prisoner at his own home by a party of Tories and Indians and taken with twenty other prisoners to Montreal, where they were sold to the British for a quantity of blankets and rum. Thomas was a prisoner at Montreal and at Prison Island, on the Saint Lawrence river 45 miles above Montreal, for about two years. At the expiration of that period he was exchanged and conveyed by water to Boston from where he made his way to his old home in Danbury, Connecticut, where he found some members of his family who had gone there after he had been taken prisoner.
From Massachusetts Soldiers and Sailors of the Revolutionary War: Barnum, Thomas, Lanesborough. Sergeant, Capt. Daniel Brown's co.; marched to Meloomscuyck Aug. 14, 1777, on an alarm; service, 6 days; also, Capt. Brown's co., Col. Benjamin Simonds's regt.; marched on an alarm at Berkshire Oct. 13, 1780; service, 6 days; also, Capt. Noah Hinman's co., Col. Asa Barns's (Berkshire Co.) regt.; marched Oct. 15, 1781; service, 10 days.
In order for the dates given for him to be correct, either Thomas would have been born when his father was 14 years old or his father's birth year is stated incorrectly.