The 1st Massachusetts Cavalry Regiment was organized at Camp Brigham in Readville, Massachusetts on September 3, 1861, under the command of Colonel Robert Williams. Initially, it was attached to the Department of the South until April 1862. The regiment's early duties included operations in South Carolina, particularly at Hilton Head and Edisto Island. The 1st Massachusetts was present at many of the significant Eastern Theater battles that involved cavalry engagements and operations. The 1st fought at the First Battle of Pocotaligo (October 22, 1862), the Battle of Antietam (September 17, 1862), the Battle of Fredericksburg (December 11-15, 1862), the Battle of Gettysburg (July 1-3, 1863), the Battle of the Wilderness (May 5-7, 1864), the Battle of Spotsylvania Court House (May 8-21, 1864), and the Siege of Petersburg (June 1864-April 1865). Perhaps the 1st Massachusetts's most successful campaign came at the Battle of Chancellorsville (April 30-May 6, 1863). The regiment participated in General George Stoneman's destruction of vital Confederate railroads that were used to access Richmond for food supplies and arms. As a result of Stoneman's efforts, Confederate General Thomas "Stonewall" Jackson could not be redirected to Richmond, where more capable doctors could tend to his wounds after he was struck on May 4th. Jackson died at Guinea Station on May 10th. In December 1863, four new companies (I, K, L, and M) were added to the regiment. They continued to serve in various capacities, including provost marshal duties and headquarters assignments within the Army of the Potomac. The regiment was mustered out of service on June 29, 1865, and discharged at Readville on July 24, 1865.