On May 16, 1861, George B. Cosby was appointed Captain and assistant adjutant general in the regular army of the Confederate States. On June 20, 1861, he was promoted to Major with the same duties. He also became assistant adjutant general of the Army of the Peninsula until the following month. Between November 9, 1861 and February 16, 1861, he was assistant adjutant general and chief of staff for Brigadier General Simon B. Buckner. As such, Cosby brought the note that opened negotiations for the surrender of Fort Donelson, Tennessee, on February 16, 1862, from Brigadier General Buckner to Union Brigadier General Ulysses S. Grant. Cosby was captured as a result of the surrender of the Fort Donelson. He was not exchanged until August 15, 1862.