Four companies were initially organized as the 9th Ohio Cavalry (1st Battalion) at Zanesville, Ohio October 3, 1862, and mustered in for a three years under the command of Colonel William Douglas Hamilton.
The 9th Ohio Cavalry Regiment completed organization at or near full strength with the 2nd Battalion organized at Camp Dennison near Cincinnati, Ohio, November 6, 1863, and the 3rd battalion also organized at Camp Dennison, December 16, 1863.
The regiment was attached to 2nd Brigade, 1st Division, XXIII Corps, Army of the Ohio, to July 1863 (1st battalion).
2nd Brigade, 4th Division, XXIII Corps, to October 1863 (1st battalion).
4th Brigade, 4th Division, XXIII Corps, to November 1863 (1st battalion).
2nd Brigade, 2nd Division, Cavalry Corps, Army of the Ohio, to March 1864 (1st battalion).
Mounted Brigade, Garrard's Cavalry Division, XXIII Corps, Army of the Ohio, to October 1864.
The 9th Ohio Cavalry mustered out of service August 2, 1865, at Lexington, North Carolina.
Moved to Camp Dennison, Ohio, December 1, 1862, and served duty there until April 1863.
Moved to Lexington, Ky., April 23, 1863, then to Manchester, Ky., and duty there until June 15.
Burnside's Campaign in eastern Tennessee August 16-October 17.
Patrol duty along the Tennessee River at Athens and Florence, Ala., until May.
Rousseau's Opelika Raid to Atlanta & West Point Railroad July 10–22.
Ten Island Ford, Coosa River, July 13.
McCook's Raid on Atlanta and West Point and Macon & Western Railroad July 27–31 (detachment).
Taylor's Hole Creek, Averysboro March 16.
A soldier from the regiment is featured in a scene in Chapter 3 of MacKinlay Kantor's Pulitzer Prize-winning novel, Andersonville (1955).