Mifflin County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 46,143. Its county seat is Lewistown. The county was created on September 19, 1789, from parts of Cumberland County and Northumberland County. It was named for Thomas Mifflin, the first Governor of Pennsylvania. Mifflin County comprises the Lewistown, PA Micropolitan Statistical Area. The county is part of the Central Pennsylvania region of the state.