Johnson County is a county located in the U.S. state of Tennessee. As of the 2020 census, the population was 17,948. Its county seat is Mountain City. It is the state's northeasternmost county, sharing borders with Virginia and North Carolina.

Mountain City is a town in and the county seat of Johnson County, Tennessee, United States. The population was 2,415 at the 2020 census. It is the northeasternmost county seat in Tennessee. In addition, at an elevation of 2,418 feet (737 m), it has the distinction of being the highest incorporated city in the state.