At the beginning of the Civil War, Patterson was the oldest Major-General by commission in the United States. In April 1861, he was mustered into service as a major general, assigned to a military department composed of Pennsylvania, Delaware, Maryland and the District of Columbia. He crossed the Potomac at Williamsport on June 15, 1861, assigned the responsibility of watching General Joseph E. Johnston's troops at Winchester, Virginia. He later claimed the failure of General Scott to send him orders, for which he had been directed to wait, caused his failure to cooperate with McDowell in the movements that resulted in the Battle of First Manassas. In July 1861, Patterson commanded the Union forces during the battle at Hoke's Run, WV. He was mustered out of service upon the expiration of his commission at the end of July.
