Mill Run is an unincorporated community and census-designated place located in Blair County, Pennsylvania, within the United States. Situated in the western part of the county, it lies south of the center of Logan Township and is bordered to the east by the city of Altoona. Although technically separate, it is often considered a neighborhood of Altoona and utilizes the city's numbering system, including avenues numbered eleven through fifteen. The community occupies the valley of Mill Run, a stream originating from the Allegheny Front two miles to the west.

The settlement was first listed as a census-designated place prior to the 2020 census, marking its formal recognition in federal demographic data. Its development is closely tied to the local geography, particularly the waterways that define the region. The stream flows southeast and then south to join Beaverdam Creek west of Hollidaysburg, establishing the area as part of the Frankstown Branch Juniata River watershed, which ultimately leads to the Susquehanna River.

The character of the place is defined by its proximity to significant natural landmarks and infrastructure. It is located near the famous Horseshoe Curve, a notable engineering feat in the region. The area also features Mill Run Reservoir, situated one mile upstream from the community, while the larger Allegheny Reservoir lies just to the west. These water bodies and the surrounding terrain contribute to the distinct physical identity of the census-designated place.

Visitor appeal is largely driven by its geographical context rather than specific cultural institutions or festivals. The proximity to Horseshoe Curve and the reservoirs offers opportunities for scenic appreciation and outdoor activities. The community's integration with Altoona means that residents and visitors have access to the broader amenities of the city, including educational facilities managed by the Altoona Area School District, which operates Altoona Area High School as its comprehensive high school.

The population of Mill Run was recorded at 231 individuals as of the 2020 census. The community relies on the infrastructure and services of the adjacent city of Altoona for many daily needs. Its location in western Blair County places it within a specific climatic zone influenced by the Allegheny Front to the west. The area remains a small, distinct residential enclave defined by its valley setting and connection to the larger urban center.