Following an Act of Congress on August 3, 1861, which reorganized the United States Army's mounted branch and consolidated all mounted regiments under a single designation, the 2nd Dragoons were officially redesignated as the Second United States Cavalry. Assigned to the First Cavalry Division of the Army of the Potomac, the regiment participated in the major campaigns of the Eastern Theater, including the battles of Antietam, Fredericksburg, Gettysburg, Chancellorsville, Spotsylvania, and Cold Harbor.
During the Battle of Fredericksburg in December 1862, Sergeant Martin Hagan led a rear-guard action during the Union withdrawal, successfully delaying advancing Confederate forces and securing the safe passage of Union infantry across the Rappahannock River. In June 1863, the regiment participated in the Battle of Brandy Station, the largest predominantly cavalry engagement fought on the North American continent. At the Battle of Gettysburg, the division—commanded by Brigadier General John Buford, a former officer of the 2nd Dragoons—conducted a vital delaying action on July 1, 1863, holding the strategic high ground of McPherson's Ridge until Union infantry arrived. The regiment was awarded 14 battle streamers and five Medals of Honor for its conduct during the war.
The Civil War marked a significant evolution in cavalry doctrine, a transformation in which the Second Cavalry actively participated. Initially utilized for piecemeal scouting and static picket duty, the regiment was eventually integrated into a consolidated, highly mobile cavalry corps capable of independent operational maneuver. They adopted repeating firearms and mastered dismounted skirmishing techniques, effectively combining the firepower of infantry with the mobility of horse-mounted troops.
During the late-war campaigns under Major General Philip Sheridan, the regiment played a decisive role in the 1864 Overland Campaign and the Shenandoah Valley campaigns, systematically disrupting Confederate supply lines and communications. They were present throughout the Appomattox Campaign, assisting in the encirclement of the Army of Northern Virginia and actively precipitating the conflict's conclusion in April 1865.