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. Youngmanstown was another name based upon common nemonclature only, but not officially. In 1827, Mifflinburg, a.k.a. Youngmanstown, and another town adjacent to the east named Greenville, a.k.a. Rhodestown,, were combined and incorporated into a borough. The first horse drawn buggy and carriage manufacturing company was established in 1845; and Mifflinburg would become the Buggy and carriage manufacturing capital of the United States in the late 19th century. There are many Victorian homes and downtown business buildings. Mifflinburg is known for The Mifflinburg Buggy Museum, Buggy Day, annual authentic Christkindle Market the second weekend of December, and German Oktoberfest.

Union County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 42,681. Its county seat is Lewisburg. The county was created on March 22, 1813, from part of Northumberland County. Its name is an allusion to the federal Union. Mifflinburg was established by legislation as the first county seat until it was moved to New Berlin in 1815. Lewisburg became county seat in 1855 and has remained so since. Union County comprises the Lewisburg, PA Micropolitan Statistical Area, which is also included in the Bloomsburg-Berwick-Sunbury, PA Combined Statistical Area. The county is part of the Central Pennsylvania region of the state.