As McClellan's army moved up the Virginia Peninsula in the 1862 Peninsula Campaign, Reynolds occupied and became military governor of Fredericksburg, Virginia. His brigade was then ordered to join the V Corps at Mechanicsville, just before the start of the Seven Days Battles. The brigade was hit hard by the Confederate attack of June 26 at the Battle of Beaver Dam Creek, but their defensive line held and Reynolds later received a letter of commendation from his division commander, Brig. Gen. George A. McCall.