Mount Joy is a borough in Lancaster County, Pennsylvania, United States. The population was 8,346 at the 2020 census, and an estimated 8,323 in 2021. The name is often shortened to "Mt Joy", as in Mencken. 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 th…Mount Joy is a borough in Lancaster County, Pennsylvania, United States. The population was 8,346 at the 2020 census, and an estimated 8,323 in 2021. The name is often shortened to "Mt Joy", as in Mencken. 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.