Lexington was built as a sidewheel steamer at Pittsburgh, Pennsylvania, in 1861 and was purchased by the War Department and converted into a gunboat at Cincinnati, Ohio, under the direction of Commander John Rodgers.

On 6 September, the two gunboats spearheaded General Ulysses S. Grant's drive to seize strategic Paducah and Smithland, Kentucky, at the mouths of the Tennessee and Cumberland Rivers.

When a large number of fresh Confederate troops threatened Grant's men, well directed fire of grape and canister from Lexington and Tyler scattered the Southern reinforcements enabling the Union soldiers to reach safety on their transports.

Accurate fire from the gunboats pounded the fort and forced Brigadier General Lloyd Tilghman, CSA, with all but four of his defending guns useless, to strike his flag.

In continuing operations the three days following the capitulation of Fort Henry, Tyler, Conestoga and Lexington swept the Tennessee for Confederate transports, seized the unfinished steamer Eastport, and destroyed a railroad bridge spanning the river.

At this juncture his gunboats dropped down the river, near the landing where his troops were collected, and opened a tremendous cannonade of shot and shell over the bank, in the direction from where our forces were approaching."

This timely support from Lexington and Tyler swung the delicate balance of forces back to the Union side and saved Grant's men from disaster.

While the Union gunboats, were capturing St. Charles, Arkansas, 17 June a direct hit exploded Mound City's steam drum scalding many of her men.

The injured crewmen were treated on Lexington as she pushed 63 miles further upriver to Crooked Point Cut-off where shallow water forced her to turn back.

Lexington, which transferred to the navy with the other ships of the Western Flotilla on 1 October 1862, participated in the joint expedition up the Yazoo River to attack Vicksburg, Mississippi from the rear.

Though the navy supplied shore bombardment from the squadron and created diversionary movements, the Union troops, hindered by heavy rains and faced by the timely arrival of Confederate reinforcements, were forced to withdraw.

After reconnaissance work and patrol duty in the Mississippi during the summer, Lexington was ordered back to the Tennessee River on 29 October to assist General Sherman at the beginning of his drive through the Confederate heartland.

Before this obstruction could be removed, word arrived from Major General Banks of his defeat at the Battle of Sabine Crossroads near Grand Encore and retreat toward Pleasant Hill.

From this point, with falling water levels and increased Confederate shore fire the gunboats would face a desperate battle to avoid being trapped above the Alexandria rapids.

The Red River, normally high until late June, had fallen so much that the gunboats could not pass over the rapids and it seemed that the better part of the Mississippi Squadron was doomed to destruction as the Union Army made plans for evacuation.

Lieutenant Bache succeeded in getting Lexington over the upper falls, then steered her directly for the opening in the dam where the furiously raging waters seemed to promise only her destruction.

She entered the gap in the dam with a full head of steam and pitched down the powerful torrent with several heavy rolls, hung for a moment on the rocks below, then reached the calm, deep water to the ringing cheers of some 30,000 voices.