A small part of Voyageurs National Park extends into its boundary, with Lake of the Woods County to its northwest.

About 10,000 years ago almost 90% of Koochiching County was covered by Lake Agassiz.

When it receded it left low areas of decayed vegetation (muskeg); as a result, three-quarters of northern Koochiching are underlain with 2 to 50 feet of peat.

[4] The name "Koochiching" comes from either the Ojibwe word Gojijiing or Cree Kocicīhk (recorded in some documents as "Ouchichiq"), both meaning "at the place of inlets," referring to the neighboring Rainy Lake and River.

Early French inhabitants gave the names Lac à la Pluie (Rainy Lake) and Rivière à la Pluie (Rainy River) to the nearby bodies of water because of the mist-like rain present at the falls of Rainy River and then to the settlement that became known as International Falls.

European settlers in Koochiching County started coming in the late 1800's.

[6] Settlers were lumberjacks and farmers and many other trades, they "suffered through isolation, illness, harsh weather, and poverty...

The county terrain consists of low rolling hills, with swampy areas where Lake Agassiz basin was deepest.

[8] The terrain slopes to the north, with its highest point on the western part of its southern border at 1,515 ft (462 m) ASL.

In 2000, George W. Bush became the first Republican to carry the county since 1972, despite narrowly losing the overall national popular vote.

Six city governments have been created, and the rest of the county consists of unorganized territories and unincorporated communities.