Clay County is a county located in the U.S. state of Florida. Its county seat is Green Cove Springs. Clay County is part of the Greater Jacksonville Metropolitan Area. Clay County was a popular destination for tourists. The therapeutic, warm springs and mild climate were major draws for visitors. Clay County has a total area of approximately 644 square miles of which 93% is land and 7% water. As of the 2000 census, there were140,814 people, 50,243 households, and 39,390 families residing in Clay County, FL.