The regiment was organized in August 1861 by Oregon Senator Edward D. Baker, composed of 15 companies instead of the standard 10. Although raised from residents of Philadelphia, it was initially designated the 1st California in deference to Baker's wishes. After his death at the Battle of Balls Bluff in November 1861, it was renamed the 71st Pennsylvania. It was paired with the 69th, 72nd, and 106th Pennsylvania regiments to form the famous Philadelphia Brigade.