[8] Mahnomen City Hall is listed on the National Register of Historic Places.

Mahnomen is in the western part of its county, along U.S. Route 59, which leads north 25 miles (40 km) to Erskine and south 36 miles (58 km) to Detroit Lakes.

State Highway 200 passes just north of the city limits, leading west 28 miles (45 km) to Ada and east 77 miles (124 km) to Walker.

According to the U.S. Census Bureau, Mahnomen has an area of 1.05 square miles (2.72 km2), all of it land.

Mahnomen is in the western part of the White Earth Indian Reservation, which includes all of Mahnomen County and parts of neighboring counties to the east and south.

There were 582 housing units at an average density of 549.1 per square mile (212.0/km2).

36.5% of all households were made up of individuals, and 16.5% had someone living alone who was 65 years of age or older.

37.4% of all households were made up of individuals, and 25.6% had someone living alone who was 65 years of age or older.