Lewistown is a borough in and the county seat of Mifflin County, Pennsylvania, United States. It lies along the Juniata River, 61 miles (98 km) northwest of Harrisburg. The number of people living in the borough in 1900 was 4,451; in 1910, 8,166; and in 1940, 13,017. The population was 8,998 at the 2000 census. It is the principal borough of the Lewistown Micropolitan Statistical Area. Of the four communities in the United States named "Lewistown", this borough is the largest.