Early life and career
Samuel Cooper was born in New Hackensack, Dutchess County, New York. He was a son of Samuel Cooper and his wife Mary Horton. In 1813 he entered the United States Military Academy at age 15 and graduated 36th in a class of 40 two years later (the customary length of study in that period.) He was appointed a brevet second lieutenant in the U.S. Light Artillery on December 11, 1815. He was promoted to first lieutenant in 1821 and to captain in 1836.
In 1827, Cooper married Sarah Maria Mason, becoming the brother-in-law of future Confederate diplomat James M. Mason and later the father-in-law of Union General Frank Wheaton. Sarah's sister, Ann Maria Mason, was the mother of Confederate cavalry general Fitzhugh Lee, a nephew of Robert E. Lee, while her brother John Mason, was a son-in-law of Gen. Alexander Macomb. Cooper served as aide-de-camp for Gen. Macomb from 1828 to 1836 and under his supervision authored A Concise System of Instructions and Regulations for the Militia and Volunteers of the United States.
Cooper served in numerous artillery units until 1837, when he was appointed chief clerk of the U.S. War Department. In 1838 he received a brevet promotion to major and was appointed assistant adjutant general of the Army. Nine years later, with a brevet as lieutenant colonel, he served in the same capacity.
Cooper's service in the Second Seminole War of 1841–42 was a rare departure for him from Washington, D.C. He was chief of staff for Col. William J. Worth, and after hostilities ended he returned to staff duty in Washington from 1842 to 1845. Cooper received a brevet promotion to colonel on May 30, 1848, for his War Department service in the Mexican–American War, and was promoted to the permanent rank of colonel in the regular army and appointed the army's Adjutant General on July 15, 1852. Cooper also served very briefly as acting U.S. Secretary of War in 1857.
Cooper was also a slave owner. At the time of the 1850 census, he owned six slaves.