Two years later, on January 22, 1862, William H. Paul enrolled for Civil War military service in Philadelphia, Pennsylvania. He then officially mustered in at Camp McClellan in Philadelphia's Nicetown-Tioga neighborhood that same day as a private with Company E of the 90th Pennsylvania Infantry. Military records at the time described him as being a 20-year-old Philadelphia resident who was 5’ 3-1/2” tall with brown hair, hazel eyes and a light complexion. At the end of March, following basic training, he was transported with his regiment by rail to Baltimore, Maryland, quartered at the Patterson Park Barracks, and equipped with an altered smooth-bore musket before being moved again, by way of Washington, D.C., to Aquia Creek Landing, and assigned with his regiment to the 2nd Brigade, 2nd Division of the U.S. Army's 1st Corps under the command of General Irvin McDowell. Marched to various sites in support of Union Army operations for the next several months, Paul and his fellow 90th Pennsylvanians saw their first major action in the Battle of Cedar Mountain, Virginia (August 9, 1862) during the opening of the Northern Virginia Campaign. Ordered to destroy a railroad bridge at Mitchell's Station on August 15, they next guarded the Union Army's rear as it retreated across the Rappahannock River, and then rendered support to Union artillery batteries during the First Battle of Rappahannock Station (August 22–August 25) before moving on to operations associated with the Battle of Thoroughfare Gap (August 28). After fighting in the Second Battle of Bull Run (August 28–30) and Battle of Chantilly (September 1), they were reassigned to the Maryland Campaign under Major-General Joseph Hooker, and then fought again in the Battle of South Mountain (September 14).