USS Lexington, a 448-ton side-wheel steamer, was built in 1860 at Belle Vernon, Pennsylvania, for commercial use. She was acquired for the Army's Western Gunboat Flotilla in June 1861 and converted to a "timberclad" river gunboat, with officers to be provided by the Navy. Assigned to duty on the Mississippi River and its tributaries, Lexington took part in several actions with Confederate vessels and land forces during the last four months of 1861 and early 1862. In February 1862, she took part in the capture of Fort Henry and in other operations on the Tennessee River. During the next few months, she continued her activities in that area, participating in the great Battle of Shiloh in early April.

In June 1862, Lexington went up the White River and helped to bombard enemy positions at Saint Charles, Arkansas. She operated on the Mississippi during much of the rest of 1862 and was formally transferred to the Navy in October. November and December found her on the Yazoo, clearing "torpedoes" and bombarding the Confederates. Lexington was active in Arkansas and Tennessee in 1863, participating in the Capture of Fort Hindman, Arkansas, in January, the defense of Fort Donelson, Tennessee, in February, and the burning of Palmyra, Tennessee, in March. In March-May 1864, she took part in expeditions up the Black, Ouachita, and Red rivers in Louisiana, and helped defeat an attack on White River Station, Arkansas, in June. Following the end of the Civil War, USS Lexington was decommissioned in July 1865 and sold the following month.