Mayo is a census-designated place (CDP) in Anne Arundel County, Maryland, United States. The population was 8,298 at the 2010 census. The Mayo CDP of 2010 includes all of the area that used to be counted as the Selby-on-the-Bay CDP. The beach in Mayo used to be a popular weekend resort. According to the United States Census Bureau, the CDP has a total area of 9.5 square miles (24.5 km2), of which 5.6 square miles (14.6 km2) is land and 3.8 square miles (9.9 km2), or 40.47%, is water.
Mayo takes its name from Commodore Isaac Mayo, who distinguished himself in the Mexican–American War. Commodore Mayo held a large tract of land on the South River, known in the early 1900s as "Mayo's Neck", that had been the home estate of Captain Nicholas Gassaway, the son of Maryland politician Colonel Nicholas Gassaway. His home, known as Gresham, was listed on the National Register of Historic Places in 1990.