In April 1861 Seymour had been posted at Charleston Harbor. He was brevetted a major for his service in the defense of Fort Sumter. In the following year he was promoted to brigadier general and assigned to the V Corps during the Peninsula campaign. After the capture of his division commander at the Battle of Glendale, Seymour took command of his division during the subsequent Battle of Malvern Hill. He served under Gen. John Reynolds at the battle of First Manassas. During the Maryland campaign, his brigade distinguished itself at the Battle of South Mountain. Seymour received a promotion to lieutenant colonel. After his service at the Battle of Antietam, he was transferred to Charleston Harbor where he led the attack on Fort Wagner in July 1863. He was severely wounded in this attack but returned to service in December. He served in the expedition to Florida, including the Battle of Olustee. In spring 1864 he was returned to the Army of the Potomacدnly to be captured at the battle of the Wilderness in May and held until August. He commanded a division in the VI Corps for the remainder of the war through the siege of Petersburg and in the Appomattox campaign.
