Colusa County was named after two Mexican land grants; Coluses (1844) and Colus (1845). The name of the county in the original state legislative act was spelled Colusi, and often in newspapers was spelled Coluse. The word is derived from the name of an Indian tribe living on the west side of the Sacramento River. By 1855 the name was established as Colusa County. It is one of the original 27 counties but was administered by Butte County until 1851. The Colusa County location almost completely changed over the years. There remains in the present county only about a six mile wide strip from the original county. That strip is in the northern boundary of the present Colusa County (the southern part of the original Colusa County). In 1850, Colusa territory corresponded to what is now roughly Glenn County. Its southern boundary was several miles north of the City of Colusa. Territory which at one time was in Colusa County is now in Butte, Glenn, Lake and Tehama Counties.

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