Steam ram Monarch was purchased at Pittsburg in April 1862 and commissioned there as part of Colonel Ellet-s Ram Fleet, Capt. R. W. Sanford in command.
After fitting out at New Albany, Monarch began active duty with the Ram Fleet. Steaming downriver in May, she scouted Fort Pillow in June and joined Benton, Louisville, Carondelet, St. Louis, Cairo and Queen of the West in the Battle of Memphis 6 June. Engaging the Confederate River Defense Fleet, the rams destroyed seven of the southern ships, wiping out the Defense Fleet as an effective naval force. Monarch rammed Colonel Lowell and General Beauregard. The Union forces took Memphis 6 June, clearing the upper Mississippi of southern forts and naval craft. On 26 June, Monarch and Lancaster pursued Gen. Earl Van Doren down the Mississippi and up the Yazoo, the Confederates burning the ship below Yazoo City to prevent her capture.
Beginning June Monarch operated against Vicksburg. Monarch and five other ships departed Helena, Ark., 16 August on an Army‑Navy expedition up to Yazoo River with troops landing at various points along the shore and destroying batteries on the river. Union ships captured CSS Fairplay above Vicksburg 27 August; Monarch then cruised the Yazoo with General Bragg to prevent Confederate use of steamer Paul Jones and to hinder communications with Vicksburg. Later that year, Monarch swept torpedoes in the Yazoo.
Monarch joined 11 other ships in the expedition to capture Fort Hindman 4 January 1863, a point that Porter noted as "a tough nut to crack." Joining efforts with Sherman's troops 9 January, the Union ships shared in the capture of Fort Hindman 11 January. In February, Monarch steamed up the Yazoo to Greeneville to relieve Commander Prichett, controlling guerrilla activity. In April, she joined Lioness, Horner, and Fulton in supporting Colonel Ellet's Marine brigade in the Tennessee.
With the fall of Vicksburg, in July, and the collapse of Confederate naval forces on the western rivers, the ram's mission was accomplished. She was dropped from the naval list in 1864, but remained in reserve, ready for recall to active service, until going to Mound City for dismantling in July 1865.