Thomas Brown was born into slavery on September 6, 1841, in Albemarle County, Virginia. In the early 1850s, his owner John Coles Carter relocated him to Prairieville, Missouri, where he performed farm labor. Brown married Phillis Goodman sometime before the war, but she died in Quincy, Illinois, while he was serving in the army.

While still enslaved, Brown enlisted as a private in Louisiana, Missouri. He mustered in to Company G of the 18th USCT Infantry Regiment at Benton Barracks in St. Louis on September 3, 1864. His service record describes him as 5 feet, 1 inch tall, with crisp hair, black eyes, and black complexion. Brown was promoted twice during the war: to corporal on November 1, 1864, and to sergeant on March 1, 1865. He was sick in the hospital in November of 1864. Brown served throughout Kentucky, Tennessee, and Alabama. He helped defend Nashville against Confederate General John Bell Hood in December 1864, pursued Hood to the Tennessee River later that month, and participated in a skirmish at Elrod's Tan Yard, Alabama, on January 25, 1865. He mustered out as a sergeant on February 21, 1866, in Huntsville, Alabama.

Brown lived in St. Louis after the war, and in 1888 he moved to Louisiana, Missouri. Brown married Mary Jane Brown on October 12, 1881, probably in St. Louis. They had one son, John Brown, who was either born on October 15, 1886, or July 4, 1889. Brown applied for a pension later in life, claiming to suffer from rheumatism and disease of the digestive organs. It is unclear, however, whether he ever received payment. He died on September 6, 1917, in Louisiana, Missouri, of unknown causes. Mary Jane Brown applied for a widow's pension, but she never received one. She worked as a housekeeper in Louisiana and passed away on December 19, 1920, due to complications surrounding dropsy and capillary bronchitis.