During the Civil War he was colonel of the 11th Pennsylvania Infantry, often rising to brigade command upon the wounding of superior officers.

[2] After leaving college in 1845 at the age of 19, he worked in the law office of his uncle, Richard Coulter (1788–1852) in Greensburg where he remained until the beginning of the Mexican–American War.

[3] Coulter saw action under General Winfield Scott in the Siege of Vera Cruz and the subsequent battles of Cerro Gordo, Contreras, Churubusco, and Chapultepec, and the capture and occupation of Mexico City in 1847.

He served directly under future Civil War general John W. Geary, a man he had little respect for due to his vanity.

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.

[6] During the siege of Petersburg Coulter briefly returned to the front in command of the 2nd Brigade, 3rd Division, V Corps at the battle of Globe Tavern.

Coulter began a partnership with George Franklin Huff, a local businessman and financier who later became a state senator and a U.S. congressman.

Coulter and Huff prospered with the rising steel industry by developing and mining the coalfields in Westmoreland County.