[3] Armagh is the oldest community in Indiana County, having been founded in 1792 by eight families led by James and Margaret Jane Graham from Northern Ireland.

The community was named after the Irish city and county of Armagh, which was the hometown of some of the settlers.

Its first school began in 1799, using a log cabin, and in 1805, it established the county's first post office.

[5] The community became a borough on April 10, 1824, having previously been part of East Wheatfield Township.

[7] Armagh was described in the 1940 Pennsylvania Guide as being "settled in 1792 by Irish immigrants, who found the hillside bare except for a few scrubby oaks; hence the name.

[9] According to the United States Census Bureau, the borough has a total area of 0.1 square miles (0.3 km2), all land.