Mustered in at Quincy, Ill., on the 1st of September, 1862, and was ordered into Kentucky on the 23d, where it was assigned to Grose's (10th) Brigade, W. S. Smith's (4th) Division, Army of the Cumberland. At Stone's River it fought in Grose's (3d) Brigade, Palmer's (2nd) Division, Crittenden's Corps (Left Wing),— its casualties amounting to 35 killed, 124 wounded, and 8 missing; total 167 out of 357 engaged, as officially reported by Colonel Waters. Although this was its first experience under fire, the regiment received gratifying mention in the official reports, and was commended for steadiness and veteran-like movements while under a terrible fire. At Chickamauga it lost 13 killed, 83 wounded, and 9 missing. While on the Atlanta campaign it served in Grose's (3d) Brigade, Stanley's (1st) Division, Fourth Corps.

After the capture of Atlanta the corps moved north through Tennessee, where it confronted the army of the Confederate General Hood. The Eighty-fourth was present at the battles of Franklin and Nashville, but its loss in these engagements was slight. While on the latter campaign, the division (First Division, Fourth Corps) was commanded by General Kimball. The regiment lost but few men taken prisoners, its missing ones were, nearly all, men who were killed. It was under fire for the last time at the battle of Nashville, December 16, 1864. After Hood's retreat the Fourth Corps was ordered to Huntsville, Ala., and thence, after a short stay, to East Tennessee. The regiment was mustered out June 8, 1865, and the men whose terms would not expire until after October 1, 1865, were transferred to the Twenty-first Illinois Infantry.