John was probably born in Washington County, Georgia, to either the William B. Roberts or James M. Roberts family. He enlisted in the Confederate army in March 1861 and served as an infantryman and a musician in several units until 1863 when he joined the Washington Guards, Company C of the 49th Regiment Georgia Volunteers. He quickly advanced to 2nd lieutenant of Company D and on his 24th birthday, 12 November 1864, was made captain of Company G.