Mustered in April 25, 1861, for three months, after which it reorganized and mustered in for three years. Leaving Cairo in October, it served in Missouri until February, 1862, when it moved with Grant up the Tennessee River to Fort Henry, and thence to Fort Donelson, where it was actively engaged in the assault, being then in Oglesby's (1st) Brigade, McClernand's Division; loss, 54 killed and 188 wounded, — a total of 242, out of 613 officers and men engaged. At Shiloh, under command of Captain Robert H. Sturgess, it lost 30 killed, 91 wounded, and 3 missing, out of 474 engaged. After the Siege of Corinth, May, 1862, the Eighth shared in Grant's Tennessee and Mississippi campaigns, prior to the investment of Vicksburg. During the Vicksburg campaign it was in Stevenson's (3d) Brigade, Logan's Division, Seventeenth Corps. At the battle of Raymond it lost 8 killed and 19 wounded; at Champion's Hill, 2 killed, 7 wounded, and 3 missing; and in the assault on Vicksburg, May 22, 4 killed and 19 wounded. The regiment remained in Mississippi during 1864, reenlisting in the meantime, and going home on its veteran furlough. On January 1, 1865, it left Memphis for New Orleans, proceeding thence, in March, to Mobile, where it was prominently engaged in the siege of that place. In the successful assault on Fort Blakely, April 9, 1865, it lost 10 killed and 54 wounded; its colors were the first on the enemy's works, the color-sergeant falling dead in the charge. In June, 1864, the recruits left in the field by the Seventeenth Illinois, upon its return home, were transferred to the Eighth. The regiment remained on duty in Louisiana and Texas until the spring of 1866, and was finally mustered out at Baton Rouge, May 4, 1866.