[6] According to the United States Census Bureau, the borough has a total area of 0.1 square miles (0.26 km2), all land.

38.7% of all households were made up of individuals, and 18.8% had someone living alone who was 65 years of age or older.

The Mifflintown borough boundaries limit the size of the community, meaning that retail and business functions necessarily spill over onto land of bordering municipalities.

Land surrounding Mifflintown serves as Juniata County's retail center as well.

It intersects the former mainline of US routes 22 and 322, which was moved in the early 1970s to a four-lane bypass lying east of the town.

The former Pennsylvania Railroad mainline tracks, now owned by the Norfolk Southern Railway, lie across the Juniata River in the adjoining town of Mifflin.

Actress Nancy Kulp (most widely known as "Miss Jane Hathaway" on The Beverly Hillbillies) is buried in the Westminster Presbyterian Church cemetery.