At the outbreak of the civil war, Todd raised a company of volunteers from Cumberland County. The unit, known as the Carlise Guards, was accepted into service of the Union Army as Company I of the 1st Pennsylvania Reserve Regiment. Todd served as Captain of Company I and was promoted to Major and third in command of the regiment. He fought with the 1st Pennsylvania reserves at the Battle of Gaines's Mill, the Battle of Second Bull Run and the Battle of South Mountain. He briefly assumed command of the regiment when Colonel Richard Biddle Roberts was assigned to brigade command at the Battle of Gaines's Mill.