Middletown is a census-designated place in Lake County, California, United States. Its population was 725 at the 2020 census down from 1,323 at the 2010 census, which was up slightly from 1,020 at the 2000 census. Middletown was given its name because it is halfway between Lower Lake and Calistoga, which is 17 miles to the south. The town was severely damaged by the 2015 Valley Fire.