Mifflin Township is in eastern Columbia County, bordered by Luzerne County on the east.  The Susquehanna River forms the northern border of the township, and the ridgeline of Nescopeck Mountain is the southern border.  The unincorporated community of Mifflinville along the Susquehanna is the main settlement in the township.  Hetlerville is a small unincorporated community in the eastern part of the township.

The township has a total area of 19.8 square miles, of which 19.2 square miles is land and 0.66 square miles, or 3.23%, is water.  Interstate 80 (Keystone Shortway) runs east and west through the township with an exit at Pennsylvania Route 339, southwest of Mifflinville.  There are approximately 27 miles of roads township roads in Mifflin.  The population was 2,457 at the time of the2016 Profiles of Pennslyvania survey, up 135 from the 2010 U.S. Census, and continues to grow steadily.  The township is primarily agricultural with a small business hub around the Interstate 80 on/off ramp.