Mount Airy is a town in Carroll and Frederick counties in the U.S. state of Maryland. It is part of both the Washington Metropolitan Area and Baltimore Metropolitan Area. As of the 2010 census the population was 9,288.
Outside the town limits, the surrounding countryside is colloquially referred to as Mount Airy or the "Four-County Area" by locals. The "Four-County Area" derives its name from the fact that the "Mount Airy" mailing area encompasses portions of Frederick, Carroll, Montgomery, and Howard counties.
The community was first settled in 1830 and continued to develop throughout the decade, beginning with the construction of house settlements along Parr's Ridge. A man named Henry Bussard built the first of these houses. This town was known for its train station on the Baltimore and Ohio Railroad (B&O) at Parr's Ridge. The railroad ran from Baltimore to Frederick, cities to the east and west, respectively, of Mt. Airy. Following 1830, the town received its official name after the suggestion of "Mount Airy" from an Irish B&O worker who complained about his freezing ears on a windy work day.