The county was created from the east half of Norman County on December 27, 1906, with Mahnomen, a former railway station town, as the county seat. The county was named for the town, the name of which is one spelling of the Ojibwe word for "wild rice". The Wild Rice River enters the county from Clearwater County and flows west through the central part of the county. The White Earth River originates
from White Earth Lake on the county's southern border and flows northwest to its confluence with the Wild Rice near Mahnomen. Terrain consists of low rolling hills, carved with drainages. The eastern part of the county is dotted with lakes and ponds and largely wooded. All non-wooded areas are devoted to agriculture where possible. The county slopes to the west and north, with its highest point near the middle of the east border, at 1,825' (556m). The county has an area of 583 square miles (1,510 km2), of which 558 square miles (1,450 km2) is land and 25 square miles (65 km2) (4.3%) is water. Mahnomen is one of 17 Minnesota savanna region counties with more savanna soils than either prairie or forest soils. Major Highways are US Highway 59, Minnesota State Highways 113 and 200. The county also maintains an airport just south of the City of Mahnomen on US Highway 59. The county includes 16 townships which are:
• Beaulieu Township
• Bejou Township
• Chief Township
• Clover Township
• Gregory Township
• Heier Township
• Island Lake Township
• La Garde Township
• Lake Grove Township
• Little Elbow Township
• Marsh Creek Township
• Oakland Township
• Pembina Township
• Popple Grove Township
• Rosedale Township
• Twin Lakes Township
Mahnomen is governed by a five member Board of Commissioners that meet the 2nd and 4th Tuesday of each month. There are three places in Mahnomen County on the National Register of Historic Places
Mahnomen County Courthouse – 1909 courthouse expanded in 1977, noted for its simple Neoclassical architecture and long service as the seat of an usual county established entirely within a Native American Reservation. Mahnomen City Hall – Distinctive 1937 municipal building with an asymmetrical design, cut fieldstone façade and sympathetic 1948 addition, emblematic of the Depression-era infrastructure sponsored by the Works Progress Administration. Mahnomen County Fairgrounds Historic District – Fairgrounds with eight contributing properties built 1936-38, representative of the importance of the county fair in rural Minnesota and the enduring output of the Works Progress Administration. There are also 12 WMA area within the county
• Beaulife State Wildlife Management Area
• Bejou State Wildlife Management Area
• Budde Meadow State Wildlife Management Area
• Dittmer State Wildlife Management Area
• Foot State Wildlife Management Area
• Hasselton State Wildlife Management Area (part)
• Loncrace State Wildlife Management Area
• Mahgre State Wildlife Management Area
• Rush Lake State Wildlife Management Area
• Santee Prairie Scientific and Natural Area
• Wambach State Wildlife Management Area
• Warren Lake State Wildlife Management Area
