The county is named for the City of Tehama. The origin of the name is not known. Suggested possible roots are the Arabic word تهامة tehama ("hot low-lands"), the Spanish word tejamanil (shingle), or "high water" in the dialect of local Native Americans.
The first permanent settlers in the area that is now Tehama County were Robert Hasty Thomes, Albert Gallatin Toomes, William George Chard, and Job Francis Dye. The four men were each given land grants by the government of Mexico in 1844. Thomes received Rancho Saucos, Toomes received Rancho Rio de los Molinos, Chard received Rancho Las Flores, and Dye received Rancho Primer Cañon o Rio de Los Berrendos. Later in the same year Josiah Belden received Rancho Barranca Colorado.
Famous early figures include Kit Carson, who took part in a fight that gave name to Bloody Island and Battle Creek, Jedediah Smith, John Fremont, and William B. Ide, the first and only president of the California Republic.
The legacy of racism in Tehama County includes the January 1886 forced removal of Red Bluff's Chinese population, followed by the August 1886 torching of Red Bluff's Chinatown by arsonists. The January 29th, 1886 edition of The Daily Alta detailed 'The Anti-Coolie Move' and confirms that a secret anti-Chinese meeting was convened in the town of Tehama, and an organization established to remove the estimated 2,000 Chinese in and around Vina. Secret daily anti-Chinese caucuses in Red Bluff were also held.
The township of Corning, notably the Maywood Colony was remarkably strident in its profession of racist practice. The Maywood Colony was a large scale agricultural development of Warren N. Woodson, which was marketed nationally through many newspapers and periodicals. An advertisement in the February 1906 edition of Better Fruit...Better Vegetables proclaimed "There are no Japs, Chinese, Italians, Negroes, or other objectionable foreign elements being mixed up with us. They are not here because they are given no encouragement. They are not employed."