McDowell initially attended the College de Troyes in France, then graduated from West Point in 1838, ranked 23 out of 45 in the class. He was commissioned a 2nd lieutenant and posted to the 1st U.S. Artillery. He was a tactics instructor at West Point for a while, before being aide-de-camp to Gen.
John E. Wool during the Mexican War. He was brevetted captain at Buena Vista.
After the war, he served in the Adjutant General's department and received a brevet promotion to major in 1856.
McDowell was promoted to Brigadier General on May 14, 1861, and given command of the Army of Northeastern Virginia, despite never having commanded troops in combat. Although he knew that his troops were inexperienced and unready, pressure from the Washington politicians forced him to launch a premature offensive against Confederate forces in northern Virginia. His strategy during the 1st Bull Run was imaginative but ambitiously complex, and his troops were not experienced enough to carry it out effectively, resulting in an embarrassing rout. After the defeat, Major Gen. George B. McClellan was placed in command of the new Union army in Virginia, the Army of the Potomac.
McDowell commanded a division in the new army, but McClellan soon reorganized his command and he was given I Corps the following spring. His corps stayed behind to defend Washington, and was eventually supposed to march to McClellan's support while the latter fought in the Peninsula Campaign. However, the nervous politicians who feared that Lieutenant Gen.Thomas J. "Stonewall" Jackson's 1862 Shenandoah Valley Campaign would eventually attack Washington kept McDowell's 40,000 soldiers behind.
Eventually, the 3 independent commands of Major Gens. McDowell, John C. Frémont, and Nathaniel P. Banks were combined into Major Gen. John Pope's Army of Virginia and McDowell led the III Corps of that army. Because of his actions at Cedar Mountain and Rappahannock Station, he was eventually brevetted a major general of the Regular Army in 1865. However, he was blamed for the subsequent Union disaster at the 2nd Bull Run. Requesting a court of inquiry, he was eventually cleared of culpability, possibly as a reward for his testimony against Major Gen. Fitz-John Porter, whom McClellan court-martialed for the defeats of the Peninsula Campaign. In 1879, when Porter's conviction was overturned, McDowell's reputation was soiled by accusations of perjury in his self-serving testimony. Partially blamed for the Union's failure, he was not given a field command for the rest of the war.
In July 1864, McDowell was given command of the Department of the Pacific. He later commanded the Department of California, the 4th Military District, the military government for Arkansas and Louisiana during Reconstruction, and the Department of the West. He was promoted to permanent major general of the Regular Army in 1872.
McDowell retired from the U.S. Army in 1882. He served as Park Commissioner of San Francisco, California, before dying in 1885. He is buried in San Francisco National Cemetery in the Presidio of San Francisco.
Brigadier General USA- May 14, 1861
Major General USV- March 14, 1862
Army and Department of Northeastern Virginia (May 27 - July 25, 1861)
Army and Department of Northeastern Virginia, Division of the Potomac July 25 - August 17, 1861)
Division, Division of the Potomac (October 3, 1861-March 13, 1862)
I Corps, Army of the Potomac (March 13 - April 4, 1862)
Department of the Rappahannock (April 4 - June 26, 1862)
III Corps, Army of Virginia (June 26 - September 5, 1862)
Department of the Pacific (July 1, 1864 - June 27, 1865)