Following the Corinth Campaign, the regiment was reassigned to General Martin E. Green's brigade, in the Confederate Army of Mississippi, Department of Mississippi and East Louisiana. Part of Green's brigade, including many from the 20th Arkansas was captured and Major Robertson was killed in the Battle of Big Black in rear of Vicksburg on May 17, 1863. The remnants of the regiment fell back inside the works and endured the forty plus day Siege of Vicksburg. The prisoners from the 20th who were captured at Big Black were sent to prison camps in the north and eventually exchanged at City Point Virginia in December 1863.