Robert Murray Morris was born in the District of Columbia on May 12, 1824, to Commodore Charles Morris and Harriet Bowen Morris. He entered the military academy of West Point, in July 1841 but resigned in January 1842. He was commissioned as Second Lieutenant in the Regiment of Mounted Rifles May 27, 1846 serving in the Mexican–American War, where he was cited for "Gallant and Meritorious conduct" in the battles of Contreras and Chapultepec.