Township municipal offices are located at 3585 US Hwy 522 North, Lewistown PA 17044.
Decatur Township lies in the southeastern corner of the county, and for forty-five years after the Cumberland County court in 1767 organized Derry Township, it was a part of it. The people living in the eastern part of Derry Township presented a petition, August, 1812, to the court of quarter sessions asking for the erection of a new township. In response the court appointed commissioners to investigate the matter and make a report. In the report, which was presented to the court at the January term in 1813, the viewers recommended the erection of a new township and closed the report by saying:
"They therefore submit to the Honorable Court the within plot or draft of Derry and the part of Beaver Dam lately annexed to it, and the division line which they have made and caused to be marked on the ground; the said line beginning at the North Boundary of Derry Township, in Jack's Mountain, and running south 25 degrees east five and a half miles to the South Boundary of the said township in the Shade Mountain, and they further beg leave to represent that by the said line the said township is equally divided, and due consideration has been paid to the local interest of said township in said division."
The report was confirmed by the court, the recommendation of the commissioners approved, and the new township ordered to be called Decatur. A year after the erection of the township the assessment rolls showed 149 land owners, eight saw-mills, two grist-mills, a fulling mill and carding machine. John Gilchrist was probably the pioneer settler in Decatur Township, having taken up 300 acres of land in the Jack's Creek Valley upon order of survey given him, January 26, 1763. The first land warrant was issued to Jacob Bach for 250 acres, August 1, 1766. On February 12, 1767, George Frey located 300 acres. Philip and William Stroup were early settlers.
George Sigler took out a warrant in 1784 for a tract of land at the head of Long Meadow run, and in 1793 was the owner of 400 acres. After the Revolution and before the close of the century the Bells, Hoffmans, Everharts, Wagners, Klines, Shillings, Yeaters, Tresters, Caleb Parshall and several other families had located in the Jack's Creek Valley, near the old Indian path that ran from the Juniata to the Susquehanna River. This path later became a public highway, over which a stage line was operated, and now is closely followed by the line of the Sunbury division of the Pennsylvania Railroad.
That part of Beaver Dam Township mentioned in the report of the viewers was made a part of Union County soon after Decatur Township was organized, but by an act of the Legislature on March 16, 1819, it was again annexed to Mifflin County and became a part of Decatur Township, where it still remains. The township of Decatur is bounded on the northeast by Snyder County; on the southeast by Juniata County; the southwest by Derry Township and on the northwest by the township of Armagh.