St. Clair, a wooden, stern-wheel, river steamer built in 1862 at Belle Vernon, Pennsylvania, was purchased on 13 August that year by the Navy Department from R. D. Cochran et al., at St. Louis, Missouri.

For many months previous, Flag Officer Foote and Commodore C. H. Davis had commanded the victorious Western Flotilla for the U.S. War Department, gaining control of the Mississippi River from Cairo, Illinois, to Memphis, Tennessee.

On 1 October 1862, the Western Flotilla was transferred to the Navy Department, as the Mississippi Squadron, and was placed under the command of Acting Rear Admiral David D. Porter on the 15th.

The objectives of the Mississippi Squadron were to cooperate with and support Generals William Rosecrans, Ambrose Burnside, Ulysses S. Grant, and others in combating guerrillas along the river banks; to stop the transport of arms and munitions from Mississippi to Arkansas; to punish rebel supporters living in and around the river communities; and to escort Federal troop and supply transports on the waterways.

St. Clair was sent up the Ohio River on 23 October 1862 to assess rebel inhabitants of Caseyville, Kentucky, for robbery of the transport, Hazel Dell, and to close the ferries and curtail cross-river communications.

Failing to receive an indemnity of 35,000 dollars from the townspeople, St. Clair's crew arrested those who could not prove loyalty and held them on board to turn them over to the Union Army.

Though firing at supposed positions and blinded by the wooded river banks, it was later found that the rebels were never able to avoid the shelling and suffered several hundred casualties.

Following repair, she departed Cairo late in June to resume escort duty for convoys bringing supplies to forces besieging Vicksburg.

Unexpected strength in the Confederate ranks, unusual low water conditions for the year, and difficulty in coordinating movement of separate large forces overland brought about failure, to some extent, for the campaign.

As they were constructing dams to raise the river to float the boats over the rocks and falls, St. Clair was ordered from Baton Rouge, Louisiana, to Alexandria to bolster defenses.

The Yazoo River now became for a short time the focal point of Porter's squadron operations, while St. Clair continued patrol and convoy duty in the lower Mississippi until August when she steamed to Mound City, Illinois, for repairs.