Before Minnesota became a state, Mahlon Black served in the territorial legislature and as mayor of Stillwater. When the Civil War began, he mustered into the Second Company of Minnesota Sharpshooters. The Company was part of an elite brigade called Berdan's Sharpshooters before joining the First Minnesota Infantry Volunteers, whom they fought alongside for much of the war. The Sharpshooters saw a lot of action. They marched through much of Virginia and were present at important fights such as the Battle of Gettysburg, the Battle of Antietam, and the Siege of Petersburg. After the war, Black moved with his wife and daughter to Minneapolis.