It was noted for its holding the high ground at the center of the line at Antietam as part of Tyndale's 1st Brigade, Greene's 2nd Division of Mansfield's XII Corps.

[1] The 66th Ohio Infantry Regiment was organized at Camp McArthur in Urbana, Ohio and mustered in for three years service on December 17, 1861,[2] under the command of Colonel Charles Candy.

[3] The regiment was attached to 3rd Brigade, Landers' Division, Army of the Potomac, to March 1862.

2nd Brigade, Shields' 2nd Division, Banks' V Corps and Department of the Shenandoah, to May 1862.

[3][2] 2nd Brigade, Shields' Division, Department of the Rappahannock, to June 1862.

2nd Brigade, 1st Division, II Corps, Army of Virginia, to August 1862.

1st Brigade, 2nd Division, II Corps, Army of Virginia, to September 1862.

1st Brigade, 2nd Division, XX Corps, Army of the Cumberland, to July 1865.

[3] The 66th Ohio Infantry mustered out of service at Louisville, Kentucky, on July 15, 1865.

Guarding trains of the army during the battles of Bull Run August 28–30.

Battle of Gettysburg (where they have a monument and markers designating their position on Culp's Hill)[3][4] July 1–3.

Pursuit of Lee to Manassas Gap, Va., July 5–24.

Skirmish at Garrison's Creek near Fosterville October 6 (detachment).

Ringgold Gap,[4] Taylor's Ridge, November 27.

Expedition from Bridgeport down Tennessee River to Triana April 12–16.

Operations on line of Pumpkin Vine Creek and battles about Dallas, New Hope Church, and Allatoona Hills May 25-June 5.