Henry Washington Benham began his military career after graduating first in his class from the United States Military Academy in 1837. He entered the Corps of Engineers where he was placed in charge of building coastal defenses in Boston and Washington, DC, earning him a promotion to captain. During the Mexican-American War, Benham served with the 9th Infantry and was slightly wounded during the Battle of Buena Vista.
In 1861, Benham was appointed to the position of chief engineer of the Department of the Ohio, and took part in General George B. McClellan’s western Virginia campaign. During the campaign, Benham led the advance guard of Union General T. A. Morris’ column in pursuit of Confederate General Robert S. Garnett to Corrick’s Ford, where Garnett was killed. As a result of this action, Benham received a promotion to Brigadier General of volunteers. He led an unsuccessful attack against Confederate forces in Secessionville, South Carolina, and was relieved of command by General David Hunter. Benham lost his title of brigadier general, but was sent to command the engineer brigade of the Army of the Potomac as a lieutenant colonel. Benham excelled in this position, and was mustered out of the military at the end of the war after being rewarded a brevet Major General.