"Neosho" was commissioned at Cairo, Illinois on 13 May 1863, but was not completed until 1 July. She left Cairo on 14 July 1863 and reached Vicksburg on 6 August, just over a month after it had surrendered after a lengthy siege. "Neosho" and other warships patrolled the Mississippi and its tributaries to prevent Confederate raiders and flying batteries from ambushing Union supply ships. One example was on 8 December 1863 when "a Confederate shore battery attacked and disabled merchant steamer "Henry Von Phul"; "Neosho" and steamed up to defend the ship and silenced the battery."