Mount Joy is a borough in Lancaster County, Pennsylvania, United States. The population was 7,410 at the 2010 census.
The name is often shortened to Mt Joy, as in Mencken (1963). However, citizens of the town often point out that this abbreviation is not proper because the town is not named for a mountain but is named after the "Good Ship" Mountjoy which famously broke a Catholic siege during the Siege of Derry. Due to the early settlement of the Protestant Scots-Irish in this region of Pennsylvania, many of the municipalities in the area were given names common to the North of Ireland such as: Derry Township, Londonderry Township, South Londonderry Township, Mount Joy Township, East Donegal Township, West Donegal Township, and Rapho Township.
Mount Joy is often named in lists of "delightfully-named towns" in Pennsylvania Dutch Country, along with Intercourse, Blue Ball, Lititz, Bareville, Bird-in-Hand and Paradise.
Mount Joy is located at 40°6′36″N 76°30′40″W (40.109895, -76.510977).
According to the United States Census Bureau, the borough has a total area of 2.3 square miles (6.1 km²), all of it land. It is considered to be part of the Susquenanna Valley.
In the 1970s, Mount Joy was chosen as the site of one of ten Decision Information Distribution System radio stations, designed to alert the public of an enemy attack. The system was never implemented and the station was not built.
The 2010 United States Census reports the following demographics for Mount Joy Borough: