Winthrop Harbor is a village in Lake County, Illinois, United States. The population was 6,742 at the 2010 census. It is located along the northern border of Illinois and is part of the Chicago metropolitan area. Winthrop Harbor is known for its North Point Marina, and is a warning demarcation point for the National Weather Service's marine warnings for Lake Michigan. With moorage over 1,500 boats, it is the largest marina on the Great Lakes. Nearly one million visitors take advantage of the marina, its associated beaches and other recreational facilities every summer.