On 11 February, the 11th Illinois moved to Lake Providence, Louisiana, where it remained until 20 April. While there, the regiment went on an expedition to American Bend on 17–28 March. On 23 April, 589 enlisted men transferred into the 11th Illinois from the 109th Illinois Infantry Regiment. From 26 April to 18 May, the regiment marched to join the Siege of Vicksburg, moving through Richmond, Louisiana, Perkins' Landing, Grand Gulf, Mississippi, and Raymond, Mississippi. The unit participated in the 19 May and 22 May assaults. At Vicksburg, the 11th Illinois was assigned to Ransom's 2nd Brigade, McArthur's 6th Division, Major General James B. McPherson's XVII Corps, Grant's Army of the Tennessee. Vicksburg fell on 4 July; during the siege, 11th Illinois lost 3 officers wounded and 40 men killed and wounded. In addition, Colonel Nevius was killed leading the 22 May assault. The regiment moved to Natchez, Mississippi, on 17 July, and subsequently participated in an expedition to Woodville, Mississippi. On 12 October the unit returned to Vicksburg where it was on duty until 29 July 1864. From September 1863 to August 1864, the regiment was assigned to the 2nd Brigade, 1st Division, XVII Corps.