At the start of the Civil War, Mansfield commanded the Department of Washington (April 27 - August 17, 1861), and was promoted to brigadier general on May 6, 1861. General-in-chief Winfield Scott recommended Mansfield for command of the volunteer army being raised in Washington D.C. that spring, but the command went to Irvin McDowell instead. Mansfield was not considered the best choice due to his relatively advanced age and skepticism of volunteer troops and the administration's push for a quick drive on Richmond that would end the war in a few months, and he also lacked a political sponsor in Washington. After the Union rout at Bull Run, George McClellan was put in command of the war effort in the Eastern Theater and Mansfield's hopes of army command were again dashed. McClellan also did not offer him a command in the newly-created Army of the Potomac. Mansfield was stationed at Hatteras Inlet, North Carolina, in October, following the battle fought there by Benjamin Butler in August. He was a brigade commander in the Department of Virginia from March to June 1862. His only combat activity during this period was the firing of coastal batteries from Hampton Roads against the ironclad "CSS Virginia" in its naval battle against the "USS Monitor" on March 9, 1862. Until the fall of 1862, Mansfield commanded the Suffolk Division of the VII Corps of the Department of Virginia in the vicinity of Suffolk.