It acquired outlands of Ramsey, Itasca and Pine Counties to its north and east.

The Mississippi River flows southward through the west central part of the county.

The county terrain consists of wooded rolling hills, dotted with lakes and ponds.

[5] The terrain slopes to the south; its highest point is Quadna Mountain, 2.5 miles (4.0 km) south-southeast of Hill City, at 1,591 ft (485 m) ASL.

29.5% were of German, 14.3% Norwegian, 13.0% Swedish, 6.2% Irish, 5.3% United States or American and 5.2% Finnish ancestry.

28.70% of all households were made up of individuals, and 14.00% had someone living alone who was 65 years of age or older.

Aitkin County voters have selected the Democratic Party candidate in 62.5% of national elections since 1960, as of 2020.