The regiment was organized on 6 June 1862 at Fort Independence (Massachusetts), as the 2nd Battalion of the 11th Infantry, one of the nine "three-battalion" regiments of regulars, each battalion containing eight companies of infantry, in contrast to the original ten regular regiments of infantry, which were organized on the traditional ten-company line. The 20th Infantry was first led by General George Sykes in the Battle of Bull Run.