[1] The United States Army purchased her at Pittsburgh, Pennsylvania, in April 1862 for service in support of Union Army operations and converted her into a ram in 1862 for service in conjunction with the Western Flotilla on the Mississippi River as part of the Army′s United States Ram Fleet under Lieutenant Colonel Charles Ellet, Jr. She was commissioned at Pittsburgh with Captain R. W. Sanford in command.

The Union forces took Memphis, Tennessee, on 6 June 1862, clearing the upper Mississippi River of Confederate forts and naval craft.

Monarch joined 11 other ships in the expedition to capture Fort Hindman, Arkansas, 4 January 1863, a point that Rear Admiral David Dixon Porter noted as "a tough nut to crack."

[citation needed] With the fall of Vicksburg in July 1863 and the collapse of Confederate naval forces on the western rivers, Monarch′s mission was accomplished.

She was laid up on the Mississippi River below St. Louis, Missouri, after July 1863[1] and was dropped from the naval list in 1864, but remained in reserve, ready for recall to active service.