Iron Mountain Lake is a city in St. Francois County, Missouri, United States. The population was 737 at the 2010 census. According to th…Iron Mountain Lake is a city in St. Francois County, Missouri, United States. The population was 737 at the 2010 census. According to the United States Census Bureau, the city has a total area of 2.08 square miles, of which 1.96 square miles is land and 0.12 square miles is water.