Newtown is a city in Sullivan County, Missouri, United States. The population was 113 at the 2020 census. Newtown was platted in 185…Newtown is a city in Sullivan County, Missouri, United States. The population was 113 at the 2020 census. Newtown was platted in 1858. A post office called Newtown has been in operation since 1887. Newtown is located in the northwest corner of Sullivan County along Missouri Route 139 and just to the west of Medicine Creek. The community of Harris is approximately 4.5 miles to the south and Lucerne is six miles to the north in Putnam County. The Chicago, Milwaukee, St. Paul and Pacific Railroad passes the east side of the location.