At the start of the Civil War, Tyler volunteered to be an aide-de-camp to Brig. Gen. Robert Patterson in April 1861. He served briefly as the colonel of the 1st Connecticut Infantry. He was appointed brigadier general in the Connecticut Militia on May 10, 1861 and commanded a division in Brig. Gen. Irvin McDowell's Army of Northeastern Virginia, with which he fought in the First Battle of Bull Run. Tyler was mustered out on August 11, 1861. Though he has been assigned a substantial portion of the blame for the Union disaster at Bull Run, he was appointed to the rank of brigadier general of volunteers on March 13, 1862. He was sent to the west and commanded a brigade in the Army of the Mississippi between May 1, 1862 and July 22, 1862 during the Siege of Corinth.