Just when that portion of Mifflin County south of the Juniata was taken into the township is not shown by the records, but when the assessment of 1768 was made the names of the people living in that territory were included, so it is probable that it was annexed soon after the township was organized.

The boundaries as thus established embraced all that portion of the present county of Mifflin lying north of the Juniata, River and part of what is now Brady Township in Huntingdon County. Early in 1767 a petition was presented to the court by the settlers north of the Juniata, asking for the erection of a new township in that district, and at the July term the court defined the boundaries of Derry Township as follows: 'Beginning at the middle of the Long Narrows thence up the north side of the Juniata as far as Jack's Narrows thence to include the valleys of the Kishacokulus and Jack's creek.' In August 1754, about a month after the purchase of the lands in the Juniata Valley from the Indians, Cumberland County organized four townships 'other side of the North Mountain.' These townships were Fannet, Aire, Tyrone and Lack. Derry Township, the oldest in the county and at one time including the entire county, was erected by the court of Cumberland County more than twenty years before Mifflin County was organized.

Derry township mifflin county 1780