Patapsco was the fourth U.S. Navy ship to bear that name. She was built by Harlan & Hollingsworth, Wilmington, Delaware; launched 27 September 1862; and commissioned 2 January 1863, with Commander Daniel Ammen in command.
Beginning in mid-July, she began her participation in a lengthy bombardment campaign against Charleston's defending fortifications. This led to the capture of Fort Wagner, on Morris Island, in early September. Fort Sumter was reduced to a pile of rubble, but remained a formidable opponent.
In November 1863, Patapsco tested a large obstruction-clearing explosive device that had been devised by John Ericsson. Remaining off South Carolina and Georgia during much of 1864 and into 1865, the monitor, or her boat crews, took part in a reconnaissance of the Wilmington River, Georgia, in January 1864 and helped capture or destroy enemy sailing vessels in February and November of that year.