Seamonson was born near Skien, Norway, on February 9, 1840 and immigrated to the United States as a child in 1842. The family initially lived in Muskegon, Michigan and then relocated to Pleasant Springs, Wisconsin in 1844. During the American Civil War, he served with the 23rd Wisconsin Volunteer Infantry Regiment of the Union Army, achieving the rank of sergeant. Conflicts he took part in include the Battle of Chickasaw Bayou, the Battle of Arkansas Post, the Battle of Grand Gulf, the Battle of Champion Hill, the Siege of Vicksburg, the Jackson Expedition, the Battle of Bayou Bourbeux, the Battle of Mansfield, the Battle of Pleasant Hill, the Battle of Spanish Fort and the Battle of Fort Blakely. Afterwards, Seamonson owned farms in Pleasant Springs.