Thomas Duncan (April 14, 1819 – January 7, 1887) was a career officer in the U.S. Army, serving as a lieutenant colonel during the American Civil War.

In 1867, he was nominated and confirmed for appointment as a brevet brigadier general in the regular army, to rank from March 13, 1865, for his service in the Civil War.

[1] He began his military career at age 13, serving as a private in the Illinois Mounted Volunteers in 1832 during the Black Hawk War.

[1] Duncan was promoted to major in the Regular Army and was transferred to the reorganized 3rd Cavalry Regiment (United States) on August 3, 1861.

He was wounded in a skirmish at Albuquerque on April 8, 1862, and was appointed brevet lieutenant colonel in the regular army for his actions in that engagement.