Andrews reached New Orleans with the last detachment of new troops on February 11, 1863. He was briefly placed in command of the 1st Brigade, 3rd Division, XIX Corps but less than a month later, in March 1863, Andrews was appointed chief of staff to Maj. Gen. Banks. In this capacity, Andrews assisted in organizing the Siege of Port Hudson, a costly operation consisting of several naval and land assaults. Confederate forces in Port Hudson, the last Confederate stronghold on the Mississippi River, ultimately surrendered on July 9, 1863 after hearing of the surrender of Vicksburg, Mississippi. Andrews personally accepted the sword of the Confederate commander of Port Hudson during the formal surrender and returned it to him as a compliment to their bravery.