Northwoods is a city in St. Louis County, Missouri, United States. The population was 4,227 at the 2010 census. According to the United St…Northwoods is a city in St. Louis County, Missouri, United States. The population was 4,227 at the 2010 census. According to the United States Census Bureau, the city has a total area of 0.71 square miles, all land.