Born in Franklin County, Ohio, McDowell graduated from West Point in 1838, and was posted to the 1st Artillery. He was a tactics instructor for awhile, before being attached to General John E. Wool's staff during the Mexican War. He was brevetted major at Buena Vista. After the war, he served in the adjutant general's department, before being commissioned for the start of the Civil War.
He was given the rank of brigadier general in May of 1861, and given command of the Army of Northeastern Virginia. Mainly, it was because of his acquaintance with Treasury Secretary Salmon P. Chase that earned him the promotion. Although McDowell knew that his troops were inexperienced and unready, pressure from the Washington politicians forced him to attack prematurely. His battle strategy during the First Battle of Bull Run was sound, but his troops were not experienced enough to carry them out effectively.
After the defeat at Bull Run, Major General George B. McClellan was placed in command of the new Union army in Virginia, the Army of the Potomac. He was given a division, but after the reorganization of the army, he was given I Corps. His unit stayed behind to defend Washington, and was eventually supposed to march to McClellan's support while the latter fought in the Peninsular Campaign; however, the nervous politicians who feared that General "Stonewall" Jackson's Valley Campaign would eventually attack Washington kept McDowell's 40,000 soldiers behind.
Eventually, the three independent commands of Generals McDowell, John C. Frémont, and Nathaniel Banks were combined into Major General John Pope's Army of Virginia. Because of his actions at Cedar Mountain, he was eventually brevetted Major General of Regulars in 1865; however, he was blamed for the disaster at 2nd Bull Run. It is speculated that he escaped culpability by testifying against Major General Fitz-John Porter, whom General McClellan had blamed for the defeats of the Peninsular Campaign.
In July 1864, McDowell was given command of the Department of the Pacific. He was formally promoted to Major General of Regulars postbellum, and retired from the US Army in 1882. He served as Park Commissioner of San Francisco, California, before dying in 1885.