Born in Strabane, Co. Tyrone, Ireland to Christoper Carleton and his wife Catherine Ball, he was the younger brother of Guy Carleton, 1st Baron Dorchester. As part of a military family, Thomas joined the British Army at a young age.
In 1753, he was an ensign in the 20th Regiment of Foot and saw action with his regiment during the Seven Years’ War. After the conclusion of the Seven Years War, Thomas Carleton served as an observer during the Russo-Turkish War of 1768–1774.
In 1776 during the American Revolutionary War, as a lieutenant colonel, he arrived in Quebec City with forces to relieve his brother, Sir Guy Carleton, the Governor General of Canada, who was besieged in the city by Continental Army troops. With the death of Lt. Col. Patrick Gordon on July 25, 1776, Col. Carleton was promoted to command the 29th Regiment of Foot. In 1777, Thomas Carleton’s nephew Christopher Carleton joined the 29th regiment as a major and served under his uncle’s command for the rest of the war. Col. Thomas Carleton did not get along well with his new commander, Frederick Haldimand, who replaced Guy Carleton in 1778 as Governor General of Canada, serving until 1786. Thomas Carleton returned to England.
In 1786, with his brother’s recommendation, Col. Thomas Carleton was appointed as the first Lieutenant-Governor of New Brunswick. Here, he helped to re-settle the many Loyalists leaving the United States. Carleton would serve as Governor of New Brunswick until his death on February 2, 1817. In 1803 he departed for England and administered from there, as he never returned to the province.
Mount Carleton, the tallest mountain in New Brunswick, is named for him.