Born in South Carolina, G. Norman Lieber was the youngest of Francis Lieber's three sons. Completing his legal education at Harvard Law School in 1859, he was admitted to the bar of New York and briefly practiced law. At the outbreak of the Civil War, he became a first lieutenant in the 11th infantry, U. S. Army, embarking on what proved to be a 40-year Army career. He served as regimental adjutant under General George B.McClellan throughout the Peninsular Campaign. He received a brevet commission forgallantry at the Battle of Gaines's Mill, Virginia, on June 27, 1862, the same day his brother Oscar, a Confederate soldier, died of wounds in Richmond. Norman saw action at the Second Battle of Bull Run, and was then appointed major and judge-advocate in the Department of the Gulf. Receiving two additional brevet commissions, he served as judge of the provost court in New Orleans until he was transferred to Washington to assist Judge Advocate General Joseph Holt, Lincoln's chief of the Bureau of Military Justice in the War Department.
After the war, Norman Lieber assisted his father in the Bureau of Confederate Archives. The charge of that office included seeking possible evidence of the complicity of Southern leaders in the Lincoln assassination plot and other criminal conspiracies. Norman afterwards served as judge advocate in various military departments, including the Department of Dakota in Minnesota, where the Army was confronting the Blackfeet and Sioux Indians. By the late 1870s, he had become a professor at West Point, teaching military law, including the laws of war. In 1884, he was appointed Acting Judge Advocate General and, in 1885, with promotion to brigadier general, became Judge Advocate General. During his tenure as head of the Judge Advocate General's Department, he presided over its expansion to accommodate the Army's increased legal needs in the Spanish-American War.