At the outbreak of the
American Civil War, Richardson immediately resigned as district attorney, and set about gathering a company of
volunteers from Chippewa County. He was offered a commission as captain of the company, but refused. He subsequently accepted commission as . The 7th Wisconsin Infantry mustered into service August 16, 1861, and went east to
Washington, D.C., for service in the
eastern theater of the war. He was promoted to captain in February 1863, and, at the time of the
Battle of Gettysburg, he had been detailed from his regiment to serve on the brigade staff of General
Solomon Meredith. During the evening after the first day of the battle, Richardson, moving alone between regiments, came upon a Confederate lieutenant and captured him. The lieutenant gave Richardson information about the Confederate plans for the second day of battle, and Richardson rushed to deliver the information to General Doubleday. Doubleday brought Richardson to the council of war, taking place that evening, where Richardson suggested locations for cannon placements. Richardson was promoted to acting lieutenant colonel on August 8, 1864, and took command of the 7th Wisconsin Infantry on December 17, 1864, following the resignation of Colonel Mark Finnicum. During the
Battle of Five Forks, in the closing weeks of the war, Richardson personally saved the life of Major General
Gouverneur K. Warren, and was wounded in the process. After the war, Warren offered to sponsor Richardson for a commission in the
regular army, but Richardson refused. Richardson was mustered out of the volunteers on July 3, 1865. Throughout the war, he was wounded six times and received three honorary brevets, including his final brevet to brigadier general, nominated by President
Andrew Johnson on January 13, 1866, and confirmed by the
United States Senate on March 12, 1866. ==Postbellum years==