The community was named after Brigadier General Hugh Mercer.

[7] According to the United States Census Bureau, the borough has a total area of 1.2 square miles (3.1 km2), all land.

There were 1,086 housing units at an average density of 876.7 per square mile (338.5/km2).

34.2% of all households were made up of individuals, and 13.3% had someone living alone who was 65 years of age or older.

The United States post office in Mercer contains a mural, Clearing the Land, painted in 1940 by Lorin Thompson.