Sponsored Topics. Callahan is a town in Nassau County, Florida, United States, adjacent to Downtown Jacksonville. The population was 962 at the 2000 census. Callahan is located at 30°33′39″N 81°49′51″W / 30.56083°N 81.83083°W / 30.56083; -81.83083 (30.560702, -81.830850).