With the outbreak of the war and the subsequent calls to arms by President Abraham Lincoln and then by the Governor of Pennsylvania, Andrew Curtin, Coulter raised a company of soldiers and was elected as their first captain. The company soon was made part of the 11th Pennsylvania Volunteers, in which Coulter was promoted to lieutenant colonel on April 26, 1861. When the regiment was reorganized as a three-year regiment on November 27, 1861, Coulter became the regimental commander until the end of the war in 1865. Coulter's regiment fought at Cedar Mountain, Thoroughfare Gap, and Second Bull Run. At the Battle of Antietam, Coulter assumed command of the 3rd Brigade, 2nd Division, I Corps when Brig. Gen. George L. Hartsuff was wounded. Returning to regimental command Coulter fought at Fredericksburg and Chancellorsville, being wounded in the former. At the battle of Gettysburg Coulter assumed command of the 1st Brigade, 2nd Division, I Corps when Brig. Gen. Gabriel R. Paul and all other ranking officers were wounded on July 1 near Oak Ridge. Coulter himself was wounded but retained brigade commanded until July 3 when he briefly turned over command before resuming command the same day.