[7] Trinity County has a rich history of Native Americans: Tsnungwe including the South Fork Hupa and tł'oh-mitah-xwe,[8] Chimariko, and Wintu.

In the first half of the 1850s the California State Legislature established that the boundaries of Mendocino and Trinity Counties was the 40th parallel north.

Fauntleroy, thereby resulting in Trinity County exercising jurisdiction two miles south of the 40th parallel north.

1871-2, p. 766), which concerned this boundary and was the act under which Fauntleroy acted under, authorized the survey of the theretofore unknown location of the 40th parallel north, stated that "the lines run out, marked and defined as required by this act are hereby declared to be the true boundary lines of the counties named herein", thereby making the law in the political code which defined the boundary as the 40th parallel north only a suggestion and not a fact.

[11] The county contains a significant portion of Shasta-Trinity National Forest and the Trinity Alps Wilderness—the second largest wilderness in California.

Census Tract 1.01 includes the communities of Douglas City, Lewiston, Trinity Center, and part of Coffee Creek and Weaverville.

The largest community by far is Mad River, with other smaller ones being Ruth, Kettenpom, and Zenia.

The county hosts many visitors, especially during summer months, for camping, backpacking, boating on the lakes, rafting/kayaking on the rivers, hunting, and fishing.

The summers tend to be clear, sunny, warm, and very dry, with little rain from June to September except for some mountain thunderstorms in the highest elevations.

The precipitation ranges from 30 to 35 inches at low elevations isolated from coastal influence, such as Big Bar, Hayfork, and Weaverville, up to 55 or 60 inches at high elevations, on the coastal side of South Fork Mountain, or where gaps in the mountain allow for precipitation to get through.

Kalmia Lake, at nearly 7500 feet in the Canyon Creek area of the Trinity Alps, is reputed to be the snowiest place in California, outpacing Lake Helen in Mount Lassen National Park, which receives 600-700 inches of snow each winter.

Average snowfall in the populated parts of the county ranges from 0-5 inches in the lower Trinity Valley to at least 100 inches in places above 4000 feet, such as Indian Valley west of Hayfork.

There is an extensive wild river and stream system, and the terrain is quite rugged and forested, with the highest point at Mount Eddy, over 9,000 ft (2,700 m).

Chancelulla Wilderness Trinity County has a mediterranean climate with very warm, dry and sunny summer days and high diurnal temperature variation due to the cool nights.

The hot afternoons form a stark contrast to the mild coastal climates of Humboldt County relatively nearby.

Trinity was a Republican-leaning county in Presidential and congressional elections until recently; now it is a tossup.

No Democrat had won the county since Jimmy Carter in 1976 until Barack Obama defeated John McCain by a 4% margin (50% to 46%) in 2008.

Trinity County is in California's 2nd congressional district, represented by Democrat Jared Huffman.

The following table includes the number of incidents reported and the rate per 1,000 persons for each type of offense.

incorporated dissolved K-12 school districts include:[48] Unified: Elementary: The population ranking of the following table is based on the 2020 census of Trinity County.