Armstrong, an 1861 graduate of the U.S. Naval Academy, was born in Georgia. He resigned his commission and joined the Confederate Navy in 1861, serving first on the CSS Sumter. He next joined the CSS Alabama, where he commanded a gun division consisting of the Blakely pivot gun and two 32-pounders. He later commanded a similar division of three guns during the battle of Fort Fisher. After the Civil War he moved to Nova Scotia, where he became a railroad executive. He died in 1904.