It was subsequently reorganized and mustered in for a three-year enlistment on June 14, 1861, under the command of Colonel George Day Wagner.
The regiment was attached to 1st Brigade, Army of Occupation, Western Virginia, July to September 1861.
Reynolds' Cheat Mountain District, Western Virginia, to November 1861.
21st Brigade, 6th Division, IV Corps, Army of the Ohio, to November 1862, 2nd Brigade, 1st Division, Left Wing, XIV Corps, Army of the Cumberland, to January 1863.
2nd Brigade, 1st Division, XXI Corps, Army of the Cumberland, to October 1863.
2nd Brigade, 2nd Division, IV Corps, Army of the Cumberland, to February 1864.
The 15th Indiana Infantry mustered out of service at Chattanooga, Tennessee on June 16, 1864.
Duty in Elkwater Valley, Virginia, July to November 1861.
Buell's Campaign in northern Alabama and middle Tennessee June to August.
Passage of the Cumberland Mountains and Tennessee River and Chickamauga Campaign August 17-September 22.