Trinity County was one of the original counties of California, created in 1850 at the time of statehood. Parts of the county were given to Klamath County in 1852 and to Humboldt County in 1853. Klamath County ceased to exist in 1876 after it was dissolved due to corruption and wide spread electoral fraud.

The county takes its name from the Trinity River, named in 1845 by Major Pearson B. Reading, who was under the mistaken impression that the river emptied into Trinidad Bay. Trinity is the English version of Trinidad.