Heintzelman graduated from West Point in 1826, ranked 17 out of 41 in his class. He spent 20 years on garrison duty, recruiting and performing quartermaster services. In the Mexican War, he was brevetted for gallantry. After serving in the Southwest, he was brevetted a Lieutenant Colonel. In 1851, he was brevetted Lieutenant Colonel for his services in the Southwest.
When the Civil War began, Heintzelman was commissioned Colonel in command of the new 17th US Infantry. Three days later, he was promoted to Brigadier General. Heintzelman led troops as they captured Alexandria, and fought at the 1st Bull Run, in which he was wounded while trying to rally his broken 3rd Division in the Union rout. On March 13, he assumed command of the III Corps Army of the Potomac and was promoted to major general in May.
In October of 1862, Heintzelman was relieved of corps command and assigned to the Military District of Washington. He remained there for 2 years and was then placed in command of the Northern Department, headquartered in Columbus, Ohio. In the last months of the war, he served on court-martial duty, and retired in 1869. He then became a businessman.
Colonel - May 14, 1861
Brigadier General USV- May 17, 1861
Major General USV- May 5, 1862
Brigadier General USA- May 31, 1862 (breveted for Seven Pines)
Major General USA- May 1862 (breveted for Williamsburg)
III Corps, Army of the Potomac (October 3, 1861- March 13, 1862)
Military District of Washington October 27, 1862- February 2, 1863)
XXII Corps and the Department of Washington (February 2- October 13, 1863)