Born in Germany, he immigrated to the United States in 1868. He served in the United States Army during the Wars with the Plains Indians as a sergeant in Company I, 5th United States Infantry. He was awarded the CMOH for his bravery in action on September 9, 1874 at Upper Wichita River, Texas. His citation reads simply "Gallantry in action", and his Medal was issued to him on April 23, 1875. In the action Sergeant Koelpin was part of an escort column of sixty officers and men protecting a wagon supply train. It was attacked by around 300 Kiowa and Comanche Indians, forcing the soldiers to defend themselves behind barricades for two days before helped arrived. He was one of eleven men (First Sergeant John Mitchell, Sergeants William de Armond, Fred S. Hay, George Kitchen, John Knox, and Frederick Neilon, Corporals John James, John J. H. Kelly, and William Morris, and Private Thomas Kelly) of the 5th United States Infantry to be awarded the Medal of Honor for their bravery in the action. His grave lain unmarked for ninety years until a Medal of Honor marker was placed in 2000.