Greencastle was founded in 1782 by John Allison, a prominent local merchant and surveyor, who laid out the town on his land, naming it in honor of his family's ancestral home in County Donegal, Ireland.

Strategically located in the Cumberland Valley of south-central Pennsylvania, Greencastle lies in Franklin County, approximately seven miles north of the Maryland border and near the crucial Interstate 81 corridor.

During the American Civil War, Greencastle experienced several incursions by Confederate forces, notably during the Gettysburg Campaign in 1863, when its citizens endured confiscation of goods and destruction of property.

The early economy of Greencastle was primarily agrarian, supported by fertile farmlands and a growing milling industry along the Conococheague Creek, processing grain for local and regional markets.

Today, Greencastle has evolved into a significant logistics and distribution hub due to its proximity to I-81, attracting numerous warehouses and trucking operations that provide substantial employment opportunities.

As of the 2020 Census, the Borough of Greencastle had a population of 4,204, reflecting a modest but consistent growth trend over recent decades, driven partly by its appeal as a quiet residential community with good access to larger job markets.

The Allison-Antrim Museum, housed in a historic 1860s building, serves as a vital repository of local history, preserving artifacts and records related to Greencastle's founding families, Civil War experiences, and agricultural heritage.

Annually, Greencastle hosts its popular Heritage Day festival, a vibrant community event that celebrates the town's history through craft vendors, food stalls, live entertainment, and historical reenactments.

The town features a well-preserved historic district with examples of Federal, Victorian, and early 20th-century architectural styles, including many brick homes and storefronts that contribute to its distinctive small-town charm.

The Greencastle-Antrim School District provides comprehensive public education, encompassing elementary, middle, and high schools that serve the borough and surrounding Antrim Township.

The availability of abundant freshwater from local springs and the Conococheague Creek was historically crucial for the town's development and continues to support its community needs.

The arrival of the Cumberland Valley Railroad in 1837 significantly boosted Greencastle's commercial development, facilitating the transport of agricultural products and manufactured goods to broader markets.

Greencastle is known for its strong community spirit, evidenced by active volunteer organizations, local civic groups, and numerous well-attended public events throughout the year.

Greencastle does not currently host any designated UNESCO World Heritage Sites, though its significant local history is meticulously preserved and celebrated through its various institutions.

The proximity to the Mason-Dixon Line positioned Greencastle as a strategic location during the Civil War, leading to its repeated occupation by Confederate forces and making it one of the northernmost towns to directly experience the conflict.