John G. Reynolds was born in New Jersey. He was commissioned as an officer in the Marine Corps on May 26, 1824.
Reynolds had fought in the Mexican War and by July of 1861 the 35-year veteran Marine was a major. He commanded a battalion of Marines at Bull Run, the only major land battle of the war in which Marines were involved.
After Bull Run Reynolds was promoted to lieutenant colonel, commanding an amphibious battalion at the capture of Port Royal, South Carolina, in November 1861, and surviving the sinking of the U.S. Transport Governor off Gerogetown, South Carolina. He went on to command the battalion at the capture of Fort Clinch, Florida in March 1862.
In 1864 the Commandant of the Corps Colonel John Harris died. Major Reynolds was the logical replacement, but Reynolds had run afoul of Harris and had been court martialled on a trivial charge. Reynolds was passed over in favor of Jacob Zeilin, and was forced into retirement.