Seymore, Truman was born on September 24, 1824 in Burlington, Vermont, United States. Son of Truman and Ann (Armstrong) Seymour.
Graduated from the United States Military Academy, 1846.
Brevetted Second lieutenant United States Army, 1846. Brevetted First lieutenant, then captain for bravery in Mexican War. Assistant professor drawing United States Military Academy, 1850-1853.
Served in Seminole War, Florida, 1856-1858. Brevetted major for gallant conduct as artillery captain, Fort Sumter, 1861. Commander training camp, Harrisburg, Pennsylvania, served in defense of Washington, District of Columbia, autumn 1861.
Commissioned brigadier general United States Volunteers, 1862, served with distinction at battles of Beaver Dam Creek, Malvern Hill and Antietam. Brevetted lieutenant colonel and colonel United States Army for gallantry at Antietam. Commander unsuccessful attack on Battery Wagner, Charleston Harbor, 1863, expedition to Florida, 1864, defeated at Olustee Station.
Taken prisoner in Battle of Wilderness, exchanged, 1864. Commander division 6th Corps, Shenadoah Valley, late 1864. Recipient 3 brevet commissions (major general Volunteers, brig. and major general United States Army), 1865.
Artillery major in coastal ports Fort Warren (Massachusetts) and Fort Preble (Maine), following Civil War. Retired, 1876.