The Borough of Mifflintown sprang from the plantation of John Harris, who in 1774, settled along the banks of the Juniata River at McClellan Ferry. Harris lived in a log house at the corner of Cedar and Front Streets, where he operated a store. The house also later served as his courtroom when he became a justice of the peace.
Harris laid out the town and eventually named it. His lands became a part of the original Mifflin County when it was established in 1789. Some wanted the new county seat to be located on Harris’ land, but the people in what is now Mifflin County objected. Harris then named his town Mifflintown, so that when the county seat was located to the west (now Lewistown), the name “Mifflintown” could not be used because it had already been taken.

When Juniata County was formed in 1831, the courthouse was built on a square block in the center of Mifflintown, left by the foresighted John Harris for the county.

The village of Mifflintown was incorporated as a borough on March 6, 1833.