Henry Dodge Dement was born in Galena, Illinois, on October 10, 1840. He was the grandson of Henry Dodge through his mother Mary. His father, John Dement, was also a prominent politician. Dement attended Rock River Seminary in Mount Morris, Illinois, then at Sinsinawa Mound College in Sinsinawa, Wisconsin, and the Dixon Collegiate Institute in Dixon, Illinois. Before he could graduate, the Civil War broke out and Dement enlisted with Company A of the 13th Illinois Volunteer Infantry Regiment. He was commissioned second lieutenant on April 20, 1861, then as first lieutenant that July. He served in the Department of the West, then later fought at the Battle of Chickasaw Bayou. His regiment fought at the Battle of Arkansas Post and participated in the Siege of Vicksburg. On February 3, 1863, Dement was named a captain. His final war duty was the Jackson Expedition; he then left the service in August 1863.