The 42nd New York Infantry was organized through the efforts of the Tammany Society and the Union Defense Committee of New York City. All ten companies of the regiment were recruited from the New York City area, gathered at Great Neck, Long Island and mustered into service in June of 1861.
Since the unit was organized by the Tammany Society, and that a large percentage of the unit were members of the Tammary organization, the 42nd New York Infantry also became known as the Tammany Regiment. Another nickname of the unit was the Jackson Guards.
Ordered to Washington D.C. the Tammany Regiment, 1019 men strong, left New York City on July 18, 1861. Throughout its 3 year enlistment, the 42nd New York fought in nearly 50 battles, included among them Antietam, Chancellorsville, Gettysburg, and the Wilderness Campaign.
In July of 1864, with its enlistment up, the unit was withdrawn form the lines. Veterans and recruits still wishing to battle on were reassigned to the 82nd New York Infantry.
| affair, Mason's Island, Md | Sept. 6, 1861 |
| Operations on the Upper Potomac, Va | Oct. 21-24, 1861 |
| engagement, Ball's Bluff, Leesburg, Harrison's Island and Conrad's Ferry, Va | Oct. 21, 1861 |
| advance on Winchester, Va | March 13-15, 1862 |
| siege, Yorktown, Va | April 5 - May 4, 1862 |
| battle, Seven Pines (Fair Oaks), Va | May 31 - June 1, 1862 |
| skirmish, Tunstall's Station, Va | June 14, 1862 |
| Seven Days Battle, Va | June 25 - July 1, 1862 |
| battle, Peach Orchard (Allen's Farm, near Fair Oaks Station), Va | June 29, 1862 |
| battle, Savage Station, Va | June 29, 1862 |
| engagement, White Oak, Swamp, Va | June 30, 1862 |
| battle, Glendale (Nelson's Farm), Frazier Farm, Charles City Cross Roads, New Market Cross Roads, and Willis Church, Va | June 30, 1862 |
| battle, Malvern Hill, Crew's Farm (Poindexter's Farm), Va | July 1, 1862 |
| Maryland Campaign | Sept. 6 - 22, 1862 |
| battle, Antietam, Sharpsburg, Md | Sept. 16 - 17, 1862 |
| reconnaisance to Charles Town, W. Va | Oct. 16 - 17, 1862 |
| operations, Loudon, Faquier, Rappahannock Counties, Va | Oct. 26 - Nov. 10, 1862 |
| battle, Fredericksburg, Va | Dec. 12 - 15, 1862 |
| Burnside's "Mud March", Va | Jan. 20 - 24, 1863 |
| Chancellorsville Campaign | April 27 - May 6, 1863 |
| battle, Mayre's Heights, Fredericksburg, VA | May 3, 1863 |
| battle, Salem Heights, Salem Church, Va | May 3 - 4, 1863 |
| battle, Bank's Ford, Va | May 4, 1863 |
| Gettysburg Campaign | June 11 - July 24, 1863 |
| battle, Gettysburg, Pa | July 1 - 3, 1863 |
| pursuit, near Mannsas Gap, Va | July 5 - 24, 1863 |
| advance, Rapidan, Va | Sept. 13 - 17, 1863 |
| Bristoe Campaign, Va | Oct. 9 - 22, 1863 |
| battle, Bristoe Station, Va | Oct. 14, 1863 |
| advance, Rappahannock, Va | Nov. 7 - 8, 1863 |
| Mine Run Campaign | Nov. 26 - Dec. 2, 1863 |
| demonstration, Rapidan River, Va | Feb. 5 - 7, 1864 |
| Wilderness Campaign | May 4 - June 12, 1864 |
| battle, Wilderness, Va | May 5 - 7, 1864 |
| combat, Laurel Hill, Va | May 8, 1864 |
| battles, Spottsylvania Court House, Laurel Hill, Ny River and Fredericksburg Road, Va | May 8 - 21, 1864 |
| combat, Po River, Va | May 10, 1864 |
| assault, the Salient, Spottsylvania Court House, Va | May 12, 1864 |
| operations, North Anna River, Va | May 26 - 28, 1864 |
| operations, Totopotomoy River, Va | May 26 - 28, 1864 |
| operations, Pamunkey River, Va | May 28 - 31, 1864 |
| battles, Cold Harbor, Va | June 1 - 12, 1864 |
| assault, Petersburg, Va | June 16, 1864 |
| siege, Petersburg and Richmond, Va | June 16 - July 13, 1864 |
| assault, Petersburg, VA | June 18, 1864 |
| engagement, Jerusalem Plank Road (Weldon Railroad), Va | June 22, 1864 |