Lt. Col. Harrison was appointed a second lieutenant in the United States Revenue Service in 1853. He was appointed second lieutenant in the Fifth (old Second) Cavalry in 185 as a reward for gallant and valuable services in Washington Territory. Having been born in Virginia, he was offered many inducements, in the way of promotion and rapid advancement, to resign his commission and join the rebellion against the United States, but he never remained loyal to the Union.

He was promoted a captain in 1861, and served in the defenses of Washington. He was engaged in the battle of Bull Run, where he won the commendation gallantry in action. He participated in the Manassas, Virginia Peninsular, and Maryland campaigns, and was engaged in numerous skirmishes and battles until the battle of Beverly Ford where he was again distinguished for gallantry but also suffered a sunstroke from which he never fully recovered. He continued to serve in less active positions throughout the war and until his death of tuberculosis at age 38.