Major Seymour commanded the 5th Regiment of Artillery and the U.S. Camp of Instruction at Harrisburg, Pennsylvania, from December 1861 to March 1862. He was Chief of Artillery for General George A. McCall's division of Pennsylvania Reserves from March 6, 1862, until July 25, 1862. On April 5, 1862, President Abraham Lincoln nominated Seymour for appointment to the grade of brigadier general of volunteers, to rank from April 28, 1862, and the United States Senate confirmed the appointment on that date. The appointment was made official on April 30, 1862.