On April 17, 1861 Field resigned as commonwealth attorney and volunteered with Culpeper county's minute men. He enlisted as a private and became an officer of Virginia's 13th Infantry. He fought at the Battle of Harpers Ferry. Promoted to the rank of major on March 23, 1861, Field served on the staff of General A. P. Hill. At the Battle of Cold Harbor he was wounded and later lost a leg at the Battle of Cedar Creek on August 9, 1862. After recovering from that wound in May 1863, Field continued his Confederate service as paymaster until April 9, 1865.