"Mississippi" arrived off Key West, Florida, to institute the blockade there on 8 June 1861, and five days later made her first capture, the schooner "Forest King" bound with coffee from Rio de Janeiro to New Orleans, Louisiana. On 27 November 1861, off Northeast Pass of the Mississippi River, she joined in capturing the British bark "Empress", again carrying coffee from Rio to New Orleans. The following spring, she joined Farragut's squadron for the planned assault on New Orleans. After several attempts, on 7 April 1862, she and successfully passed over the bar at Southwest Pass, the heaviest ships ever to enter the river to that time.