Winthrop Harbor is a village in Lake County, Illinois, United States. The population was 6,670 at the 2000 census, and it is the northernmost city in Illinois. Winthrop Harbor's main claim to fame is North Point Marina. 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.