The 12th Pennsylvania was raised in Pittsburgh in April 1861 for a three-month term in response to President Abraham Lincoln's call for 75,000 men, under the supervision of Brigadier General James S. Negley.

It was reviewed by state Governor Andrew Gregg Curtin in the afternoon alongside the 13th Pennsylvania Infantry before being mustered into Federal service.

Conditions at Camp Scott, described by Samuel Penniman Bates' postwar official history as a "field of mud," made many of its men eager for action.

[3] Regimental quartermaster James A. Ekin became a brevet brigadier general by the end of the war and served on the military tribunal that tried the Lincoln assassination conspirators.

[6] Private Charles Oliver of Company G later reenlisted in the 100th Pennsylvania Infantry and was awarded the Medal of Honor for his actions at the Battle of Fort Stedman in 1865, having served through most of the war.