Sumner County is a county located in the U.S. state of Kansas. As of the 2010 census, the population was 24,132. Its county seat is Wellington.HistoryEarly historyFor many millennia, the Great Plains of North America was inhabited by nomadic Native Americans. From the 16th century to 18th century, the Kingdom of France claimed ownership of large parts of North America. In 1762, after the French and Indian War, France secretly ceded New France to Spain, per the Treaty of Fontainebleau. In 1802, Spain returned most of the land to France, but keeping title to about 7,500 square miles.In 1803, most of the land for modern day Kansas was acquired by the United States from France as part of the 828,000 square mile Louisiana Purchase for 2.83 cents per acre. In 1848, after the Mexican-American War, the Treaty of Guadalupe Hidalgo with Spain brought into the United States all or part of land for ten future states, including southwest Kansas. In 1854, the Kansas Territory was organized, then in 1861 Kansas became the 34th U.S. state.