Robert Selden Garnett was born December 16, 1819, on the family plantation Champlain (also known as Font Hill) in Essex County, Virginia. He was one of seven children born to Robert Selden Garnett Sr. and Olympia Charlotte DeGouges Garnett. Robert was a cousin of, who also became a Confederate general and died during at the (1863). Robert Garnett Jr. attended the Norfolk Academy, where he concentrated on engineering, drawing, and horsemanship. He entered the U.S. Military Academy at West Point, New York, on September 1, 1837. He graduated on July 1, 1841, ranked twenty-seventh in a class of fifty-two cadets. Garnett received a commission as brevet second lieutenant in the 4th United States Artillery and soon reported to Company G of that regiment, then located near the Canadian border. On January 31, 1842, he received promotion to the rank of second lieutenant.
Although still a relatively inexperienced officer, Garnett received an appointment on July 5, 1843, as assistant instructor of infantry tactics at West Point. He held that position until October 17, 1844, when he resigned and became a recruiter. Garnett joined the staff of Brigadier General John E. Wool on January 1, 1845, at Troy, New York, and served in this capacity until September 30. At that time, he went to Fort Monroe, Virginia, and rejoined the 4th U.S. Artillery. With the regiment, Garnett went to Aransas Bay, Texas. He soon became a part of Major General Zachary Taylor’s Army of Occupation at Corpus Christi and served as adjutant for the Artillery Battalion. Garnett participated in the Battles of Palo Alto and Resaca de la Palma on May 8 and 9, 1846. Taylor named him as one of his aides-de-camp on June 29, and Garnett received promotion to the rank of first lieutenant on August 18. He distinguished himself in the Battles of Monterey (September 21–23, 1846) and Buena Vista (February 22–23, 1847). As a result, Garnett was brevetted captain and then major.
Taylor left Mexico for New Orleans, Louisiana, on November 26, 1847, and he reached his home in Baton Rouge on December 5. Garnett appears to have accompanied the general on this trip. On August 31, 1848, Garnett received a transfer to the 7th United States Infantry Regiment, though he continued to act as something of a military advisor to Taylor. Having been elected president of the United States in November 1848, Taylor resigned from the army on January 31, 1849. During the spring of that year, Garnett carried dispatches to San Francisco for General Persifor F. Smith, military governor of California. During this trip, Garnett sketched a design that was later adopted as the Great Seal of the State of California.
Garnett’s association with Taylor ended with the president’s death in July 1850. He received orders from the secretary of war later in the year to escort a group of nearly one hundred Seminole Indians from Florida to the Indian Territory. After his return to the East, Garnett began serving as a member of the Board to Revise the Uniform Dress of the Army at Washington, D.C. He received promotion to the rank of captain in the 7th Infantry on March 9, 1851. Later in the year, Garnett went on frontier duty at Corpus Christi and then at Ringgold Barracks in Texas.
Colonel, superintendent of the U.S. Military Academy, recommended Garnett as commandant of cadets and instructor of infantry tactics at West Point. Garnett assumed that position on November 1, 1852. U.S. secretary of war dismissed Garnett from this post on July 31, 1854. Garnett then returned to . Promoted to captain of the 1st United States Cavalry on March 3, 1855, Garnett saw little service in that capacity, because on March 27, he was appointed major of the newly reconstituted 9th United States Infantry.
Garnett went with his regiment to Fort Steilacoom in the Washington Territory. In August 1856, he supervised the construction Fort Simcoe. Garnett returned to the East in November and married Marianna E. Nelson of Boston, Massachusetts, on January 24, 1857. A son, Arthur Nelson, was born to the couple in February 1858 after they returned to Fort Simcoe. Marianna and Arthur died of fever on September 17 and 23, respectively. Garnett took the bodies to New York, where they were buried in Green-Wood Cemetery in Brooklyn. He then went on a leave of absence in Europe that lasted until March 1861.
After theadopted the on April 17, 1861, Garnett resigned from the United States Army. The resignation was accepted on April 30, 1861. Robert E. Lee, then commanding Virginia’s Provisional Army, recommended that Garnett be promoted to colonel and named adjutant general. Governor did so on April 25. Appointed brigadier general in the Confederate army on June 6, Garnett went to to assume command of the Department of Northwestern Virginia. He had as his mission to prevent further Union incursions and to protect the vital rail lines and turnpikes in the region. On July 13, his forces engaged Union troops in the Battle of Corrick’s Ford. The Confederates were forced to retreat, and Garnett remained near his rear guard. A Union bullet soon hit him in the back and mortally wounded him, making him the first Confederate general killed in the Civil War. The Union troops who found his body had it transferred to his family in Baltimore. On August 28, 1865, his remains were interred next to those of his wife and son in Green-Wood Cemetery.