Johnstown is a census-designated place in Fremont County, Wyoming, United States. The population was 242 at the 2010 census. Johnstown is located at 43°7′43″N 108°41′25″W / 43.12861°N 108.69028°W. According to the United States Census Bureau, the CDP has a total area of 32.1 square miles, of which 31.5 square miles is land and 0.7 square mile …Johnstown is a census-designated place in Fremont County, Wyoming, United States. The population was 242 at the 2010 census. Johnstown is located at 43°7′43″N 108°41′25″W / 43.12861°N 108.69028°W. According to the United States Census Bureau, the CDP has a total area of 32.1 square miles, of which 31.5 square miles is land and 0.7 square mile is water.