Irvin McDowell graduated from the United States Military Academy in 1838, and taught tactics at the academy from 1841 to 1845 to many of the generals he would eventually face on the battlefield. He served as an aide-de-camp to General J. E. Wool during the Mexican-American War, and received a brevet promotion to Captain for his service at the Battle of Buena Vista.
At the outbreak of the Civil War, on May 14, 1861, Irvin McDowell was promoted to the rank of Brigadier General as a result of political connections to the Secretary of the Treasury, Salmon P. Chase. Although he had never before commanded troops in combat, McDowell, facing political pressure, attacked Confederate forces at the First Battle of Manassas where he was consequently defeated. After the defeat, General George B. McClellan was made commander of the newly formed Army of the Potomac, and McDowell was detached from the army to protect Washington, DC. McDowell would later command a corps at the Second Battle of Manassas, which resulted in another defeat. On July 1, 1864, McDowell was put in command of the Department of the Pacific, and was relatively inactive for the remainder of the war.