Tennessee Code Title 5. Counties § 5-6-102

(1) In counties of Class 1 and counties of Class 2 having a county council form of county government as established by § 8-24-101, in counties having a population of not less than twelve thousand three hundred fifty (12,350) nor more than twelve thousand three hundred seventy-five (12,375), or not less than thirty-eight thousand eight hundred (38,800) nor more than thirty-eight thousand nine hundred (38,900), by the federal census of 1970 or any subsequent federal census, and in counties having a chair of the county court on May 11, 1978, the county mayor shall be elected in the regular August election in 1978 and every four (4) years thereafter.

(2) In all other counties, the county mayor shall be elected in the regular August election in 1978 and every four (4) years thereafter.