USS Monadnock, first of a two-ship class of 3295-ton twin-turret monitors, was built at the Boston Navy Yard. Commissioned in October 1864, she was sent to Norfolk, Virginia, to begin her Civil War service. In December 1864 and January 1865, she used her four fifteen-inch guns to support the two assaults that finally captured Fort Fisher, North Carolina, thus closing the port of Wilmington to blockade running. After Fort Fisher was taken, Monadnock went to Charleston, South Carolina, to take part in final operations against that city and its defenses. In April 1865, she served briefly on Virginia's James River, then steamed south to Havana, Cuba, where she remained until June, covering the Confederate ironclad Stonewall.

After special outfitting at the Philadelphia Navy Yard, in October 1865 Monadnock began a long voyage to California, the longest cruise that a monitor-type warship had yet undertaken. After calling at several South American ports and passing through the Strait of Magellan, she arrived at San Francisco in June 1866 and was soon thereafter decommissioned at the Mare Island Navy Yard. In 1874 her wooden hull was broken up as part of a program to "rebuild" Civil War era monitors into modern ones. In fact, she was replaced by a completely new ship, which was also named Monadnock.