Newfolden is a city in Marshall County, Minnesota, United States, along the Middle River. The population was 352 at the 2020 census. Old Mi…Newfolden is a city in Marshall County, Minnesota, United States, along the Middle River. The population was 352 at the 2020 census. Old Mill State Park is nearby. A post office called Newfolden has been in operation since 1896. The city was named after the old Folden Municipality in Norway.