Greencastle is a borough in Franklin County in south-central Pennsylvania, United States. The population was 3,996 at the 2010 census. Greencastle lies within the Cumberland Valley of Pennsylvania.

James Patton, who came to America at age 17 and moved to North Carolina in 1793, started the agreement of Canogege (spelled “Conegoge” by George P. Donahoo). Patton said in an 1839 letter to his descendants that the place was “settled by a moral and orderly people.”

Greencastle was founded in 1783 by John Allison from the Barkdoll House. The town was named after Greencastle, County Donegal, Ireland. It was originally composed of 246 lots. By 1790 there were virtually 60 houses in Greencastle, homes to nearly 400 people. The town of Greencastle had grown by the mid-nineteenth century to 1,125 residents.

In 1845, following the agreement crisis in the Latter Day Saint movement, Sidney Rigdon (one of the three main contenders along when James Strang and Brigham Young for leadership of the Latter Day Saints later the death of Joseph Smith) took his cronies to Pennsylvania and formed a Rigdonite Mormon deal at Greencastle. This deal had nearly 200 followers. They founded the New Jerusalem unity between Greencastle and Mercersburg, published the Conochoheague Herald newspaper in Greencastle, and made plans for the construction of a temple. The Rigdonite Mormon unity at Greencastle on your own lasted a few years; some former Rigdon followers went to Utah to associate Brigham Young, while William Bickerton, who had opposed Rigdon’s move to Greencastle, would eventually revolutionize the remaining Pennsylvania branch of the Latter Day Saint occupation in Pittsburgh as the Church of Jesus Christ (Bickertonite).