[3] At the start of the Civil War, Mansfield commanded the Department of Washington (April 27 – August 17, 1861),[4] and was promoted to brigadier general on May 6, 1861.

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.

Mansfield was stationed at Hatteras Inlet, North Carolina, in October, following the battle fought there by Benjamin Butler in August.

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.

[3] During the Maryland Campaign, when Confederate General Robert E. Lee invaded the North for the first time, Mansfield was given command of the XII Corps of the Army of the Potomac, as of September 15, 1862, two days prior to the Battle of Antietam.

On the morning of September 17, 1862, the I Corps under Maj. Gen. Joseph Hooker attacked from the north, parallel to the Hagerstown Turnpike, smashing into the Confederate left flank.

As the lead brigade moved through an open field east of the Miller farmstead, they were subjected to fire by Confederate gunners, who took a terrible toll on the rookie soldiers.

The troops were advancing in column formation, more suitable for marching, and their officers ordered deployment into open battle lines, which would reduce the risk of casualties from artillery shelling.

Mansfield countermanded these orders, insisting they stay in column, because he was concerned that outside of the immediate control of their officers, the men would break and run.