The regiment was attached to Defenses of Baltimore, Maryland, VIII Corps, Middle Department, to January 1863.
Unattached, Scammon's Division, Department of West Virginia, to December 1863.
The 18th Connecticut Infantry mustered out of service June 27, 1865, at Harpers Ferry, West Virginia.
The regiment left Connecticut for Baltimore, Maryland, August 22.
Moved to Winchester, Virginia, and joined Robert H. Milroy's command May 22.
Provost duty at Hagerstown, Maryland, September 30, and at Martinsburg March 1864.
Moved to Camp Piatt, then to Parkersburg, Cumberland, Martinsburg, Harpers Ferry, and Snicker's Ford July 1–18.