Mount Airy is a town located in Carroll, Frederick, Howard, and Montgomery 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 community encompasses portions of Frederick, Carroll, Montgomery, and Howard counties.