The southern terminus of SR 99 is located at an intersection with U.S. Route 31 in northeast Athens. From this point, the highway travels concurrently with Elm Street in a westerly direction through the city. As it exits the city limits, SR 99 changes routes, becoming an extension of Market Street before heading in a northwesterly trajectory towards Mount Rozell. From there, it then travels in a westerly direction before terminating at SR 207 northeast of Anderson.