Mifflinburg is a borough in Union County, located in the Susquehanna River Valley of central Pennsylvania, United States. In 1792, Mifflinburg was settled by Elias and Catharina Youngman and 8 other families, many of them German, and the village of Mifflinsburgh was formed. It was named for Thomas Mifflin, who was governor of Pennsylvania and first governor under the PA Constitution of 1790, and a signer of The Declaration of Independence.