Marcy, Randolph Barnes was born on April 9, 1812 in Greenwich, Massachusetts, United States. Son of Laban and Fanny (Howe) Marcy.
Graduated from the United States Military Academy, 1832.
Brevetted Second lieutenant, 5th Infantry, United States Army 1832, second in command Lieutenant, 1835, First lieutenant, 1837, captain, 1846. Acting inspector general Department of Utah, 1856. Commissioned major, appointed paymaster, 1859, served in N.W. until 1861, then chief-of-staff to General George B. McClellan (his son-in-law).
Commissioned colonel, inspector general, 1861. Temporary brigadier general, 1861-1863, served with Army of Potomac in Maryland. campaign, 1862-1863. Participated in cavalry charge at Brandy Stations in Pennsylvania campaign, 1863.
Distinguished himself at Battle of Gettysburg, in central Virginia operations and actions at Rapidan, Auburn and New Hope Church. Inspector in various departments, 1863-1878. In charge 2d cavalry div.
Army of Potomac in Richmond campaign, 1864-1865. Appointed inspector general with rank brigadier general United States Army, 1878.