In 1861 Sickel volunteered for the Union army and was appointed captain (May 27) then colonel (July 28) of the 3rd Pennsylvania Reserves. Sickel and his regiment joined the Army of the Potomac on the Virginia Peninsula in time for the Seven Days Battles. He led his regiment credibly at Mechanicsville and Gaines' Mill, receiving commendations from his brigade and division commanders, George G. Meade and George McCall. He continued in command of the regiment through the battles of Second Bull Run and Fredericksburg. Shortly after Fredericksburg, he succeeded Meade in command of the Pennsylvania Reserve Division. Sickel and his division were stationed at Alexandria, Virginia on guard duty. While many of the Pennsylvania Reserves rejoined the Army of the Potomac for the Gettysburg campaign, Sickel stayed behind as commander of a brigade in the Department of Alexandria.