John H. Haight and his younger brother, George, became two of New York's earliest responders to President Abraham Lincoln's call for volunteers to defend Washington, D.C. following the mid-April 1861 fall of Fort Sumter to Confederate States Army troops. After his brother mustered in for a three-year term of service as a private with Company E of the 72nd New York Volunteer Infantry Regiment on June 20, 1861, John Haight then also enlisted, at the age of 22, for a three-year term on July 17, 1861 in Westfield, New York. Also officially mustering in with the 72nd New York Infantry, John Haight did so on July 24 as a private with the regiment's G Company. He was then promoted to the rank of corporal on September 1. Initially under the command Colonel Nelson Taylor, the Haight brothers and their regiment were transported to Washington, D.C., where they assisted in the defense of the city before participating in the Union Army's expedition to lower Maryland (September 15–October 2, 1861). They were then reassigned with Sickles' Brigade to the U.S. Army of the Potomac in October as part of the division commanded by Brigadier-General Joseph Hooker. Their next significant deployment came as part of the U.S. Army's Third Corps, Second Division when they engaged in the Union Army's advance on Manassas, Virginia (March 10, 1862), the expedition from Dumfries to Fredericksburg, capture of stores (March 18), and reconnoissance from Liverpool Point to Stafford Court House and subsequent operations there (April 4).