When the Civil War broke out, Richardson was still farming in Michigan. He enlisted in the Union Army and recruited and organized the 2nd Michigan Infantry. He married Fannie Travor on May 18, 1861, in Wayne County, Michigan. When he reported with his regiment in Washington, DC, General Winfield Scott greeted him with "I'm glad to have my 'Fighting Dick' with me again". Richardson, promoted to colonel on May 25, 1861, was assigned command of the 4th Brigade, 1st Division, in the newly organized army of Brigadier General Irvin McDowell. His brigade saw limited action at the First Battle of Bull Run near Blackburn's Ford, and in covering the subsequent federal withdrawal to Washington. He was promoted to brigadier general, ranking from May 17, 1861.