Mountain City is a city in Hays County, Texas, United States. The population was 648 at the 2010 census.

Hays County is a county in the central portion of the U.S. state of Texas. It is part of the Austin-Round Rock metropolitan area. As of the 2020 census, its official population had reached 241,067. The county seat is San Marcos. Hays, along with Comal and Kendall Counties, was listed in 2017 as one of the nation's fastest-growing counties with a population of at least 10,000. From 2015 to 2016, Hays County, third on the national list, had nearly 10,000 new residents during the year.