Summersville is a town in Shannon and Texas counties in the U.S. state of Missouri. The population was 453 at the 2020 census. A post office c…Summersville is a town in Shannon and Texas counties in the U.S. state of Missouri. The population was 453 at the 2020 census. A post office called Summersville has been in operation since 1871. The community has the name of Jesse and Thomas Summer, pioneer citizens.