New London is a city in Ralls County, Missouri, United States. The population was 943 at the 2020 census. It is the county seat of Ralls County. New London is part of the Hannibal Micropolitan Statistical Area. William Jameson, claimed land, made a plat, and founded New London in 1819. New London was named the county seat in 1820. The Ralls County Courthouse in New London was built in 1858 and is the oldest court house in Missouri.