Lewistown is a borough in and the county seat of Mifflin County, Pennsylvania, United States. It is the principal city of the Lewistown, PA Micropolitan Statistical Area, which encompasses all of Mifflin County. It lies along the Juniata River, 61 miles northwest of Harrisburg. The number of people living in the borough in 1900 was 4,451; in 1910, 8,166; in 1940, 13,017; and in 2000, 8,998. The population was 8,338 at the 2010 census. Of the four communities in the United States named "Lewistown", this borough is the largest.