[5] Prior to the arrival of Europeans in the region, varying cultures of Native Americans inhabited the county for thousands of years.

At the time of European encounter, the Modoc people lived in what is now northern California, near Lost River and Tule Lake.

[6]: 216 The northern boundary of California, and eventually Modoc County, had been established as the 42nd parallel since the time of Mexican possession.

In the absence of a reliable survey of the 120th meridian, the eastern boundary of northern California was a subject of contention before Modoc County formed.

At the time, the Warner Mountains were believed to be a part of the Sierra Nevada, so this would have included Surprise Valley, but California denied the request.

Harassed by the Klamath, traditional competitors, a band of Modoc led by Captain Jack returned to California and the Tule Lake area.

From strong defensive positions in the lava tubes, 52 Modoc warriors held off hundreds of US Army forces, who called in artillery to help.

Settlement of the county began in earnest in the 1870s, with the timber, gold, agriculture, and railroad industries bringing most of the settlers into the area.

Early settlers included the Dorris, Belli, Essex, Scherer, Trumbo, Flournoy, Polander, Rice and Campbell families.

During World War II, the US government developed several thousand acres just south of Newell as a Japanese American internment camp.

Tule Lake War Relocation Center was the site of temporary exile for thousands of Japanese-American citizens, who lost most of their businesses and properties where they had formerly lived in coastal areas.

On November 8, 2005, Senator Dianne Feinstein called for the camp to be designated a National Historic Landmark.

The area's tumultuous seismic past has set the stage for a productive, diverse regional ecology today; a rich soil composition, largely created by ancient volcanoes dispersing vast amounts of minerals, stands out from the typically bland, clay-heavy terrain common in the American west.

To the northwest of the county lies Medicine Lake—the largest shield volcano on the U.S. West Coast—and Lava Beds National Monument.

The southwestern quadrant of the county is a unique ecosystem of isolated hardwoods (oaks) and volcanic mountains with intermountain river valleys.

Nearly 1 million acres (4,000 km2) of the Modoc National Forest lie on the plateau between Medicine Lake, to the west, and the Warner Mountains, to the east.

As it is situated within the biodiverse California Floristic Province, many native trees are found in the county, including Garry oak (Quercus garryana) and Washoe pine (Pinus washoensis).

[15] The rich plant life supports substantial populations of mule deer (Odocoileus hemionus), Rocky Mountain elk (Cervus canadensis) and pronghorn (Antilocapra americana), as well as several herds of wild horses (Equus ferus).

Some of these are retirees who have sold their houses for large profits in other parts of the state, using the proceeds to live on, while others are remote workers.

This sudden rise in housing prices become unaffordable for locals, who find themselves unable to purchase homes given their limited incomes.

[33] Federally, Modoc County is in California's 1st congressional district, represented by Republican Doug LaMalfa.

On September 24, 2013, the Modoc County Board of Supervisors voted 4–0 in favor of secession from California to form a proposed state named Jefferson.

[39] The following table includes the number of incidents reported and the rate per 1,000 persons for each type of offense: Additionally, the eastern Modoc County communities of Eagleville, Cedarville, Lake City, and Fort Bidwell are connected via Surprise Valley Road, which runs from the southern county line to the Oregon border.