Mifflin Township was officially created on 3 September 1810. Its original area (green on map above) included one other present-day township [Washington Township] which was later divided from the southern part of Mifflin Township.
Present-day Mifflin Township is within the black boundary-lines marked as such on the above map.
Uniontown Borough, was incorporated in 1864 as a separate government entity within the boundaries of Mifflin Township. In 1965, the residents of Uniontown voted to change the name of the borough to Pillow in 1965. Berrysburg Borough was incorporated in 1869 as a separate government entity within the boundaries of Mifflin Township.
The following explanation is from William Henry Egle‘s History of the Counties of Dauphin and Lebanon of the Commonwealth of Pennsylvania, published in Philadelphia in 1883 by Everts and Peck:
At a Court of Quarter sessions held 22 October 1818, an order was issued to three commissioners to inquire into the propriety of dividing the townships of Upper Paxtang [Paxton] and Lykens Township, then embracing the whole of Lykens Valley, into three townships, as had been petitioned for by the inhabitants of said valley. The commissioners reported that they were of opinion that the two aforesaid townships ought to be divided into three, and that they had accordingly run and marked the lines of division as follows, to wit:
“The First township, beginning at Mahantango Creek, a short distance from Miller’s fording, on the Old Sunbury road; thence down the same to the river, and across said river to the mouth of West Mahantango Creek; thence down the western shore of Susquehanna [River] to a point opposite to Berry’s Mountain; thence by Halifax Township across said river and along the summit of said mountain 5 miles 200 perches to a chestnut-oak tree; thence north 7 1/2 west 6 miles 80 perches to the beginning, which we have called Upper Paxtang [Paxton] Township.
“The Second we have called Berry Township, beginning at the chestnut-oak aforesaid; thence by the summit of Berry’s Mountain aforesaid 4 miles 260 perches to a small chestnut-oak; thence north 6 west 7 miles through Hain’s Gap, to Mahantongo Creek aforesaid; thence down the same to the point aforesaid, near Miller’s fording; thence by the line of Upper Paxtang [Paxton] aforesaid, reversed south 7 1/2 degrees east 6 miles 80 perches to the beginning.
“The Third, beginning at the same chestnut-oak aforesaid; thence along the summit of Berry’s Mountain aforesaid 10 miles to the line of Schuylkill County; thence by the same 8 1/2 miles to Mahantongo Creek aforesaid; thence down the same to Berry Township aforesaid; thence by the same south 6 degrees east 7 miles to the beginning and which we have called Lykens Township.”
This report was confirmed by the court 12 March 1819, except so far as the new or centre division, which the court ordered to be called Mifflin Township instead of Berry, as suggested by the commissioners. It was named for General and Governor Thomas Mifflin. (See Sess. Doc. 1815-23, page 212)
Much of the history of the township is identified with the history of the Lykens Valley proper….
The new township was originally bounded on the north by Northumberland County, on the east by Lykens Township, on the south by Halifax Township, and on the west by Upper Paxton Township.
In 1823, Halifax Township was diminished by the creation of Jackson Township, and thus, beginning in 1823, Mifflin Township was bounded on the south by Jackson Township
Mifflin Township was diminished in 1846 by the creation of Washington Township which since 1846, has bounded Mifflin Township on the south.