The 27th Pennsylvania Infantry was originally organized in Philadelphia, Pennsylvania as a state militia regiment in January 1861 as part of the "Washington Brigade" under the command of Colonel William F. Small.

The regiment was attached to 1st Brigade, Miles' Division, McDowell's Army of Northeastern Virginia, to August 1861.

Stahl's Brigade, Blenker's Division, Army of the Potomac, to March 1862.

1st Brigade, Blenker's 2nd Division, II Corps, Army of the Potomac, March 1862.

1st Brigade, Blenker's Division, Department of the Mountains, to June 1862.

1st Brigade, 1st Division, I Corps, Army of Virginia, to September 1862.

1st Brigade, 1st Division, XI Corps, Army of the Potomac, to October 1862.

2nd Brigade, 2nd Division, XX Corps, Army of the Cumberland, to May 1864.

Operations at Welford's, Kelly's and Beverly Fords April 14–15.

Duty on line of the Rapidan, near Bristoe Station, until September.

March along Nashville & Chattanooga Railroad to Lookout Valley, Tenn., October 25–28.