[9] The county was named after the 1844 Rancho Colus Mexican land grant to John Bidwell.

[10][11] The word is derived from the name of a Patwin village known as Ko'-roo or Korusi located on the west side of the Sacramento River on the site of the present-day city of Colusa.

[9] Present-day Colusa County was originally home to the Patwin band of the Wintun people,[14] whose territory included areas along the Sacramento River as well as lands extending west towards Lake County, bounded in the north by the sources of Stony Creek near Stonyford and in the south by Putah Creek.

[15] Linguistically, the Patwin people in the Colusa area spoke two dialects of the Southern Wintuan language.

The 2010 United States census reported that Colusa County had a population of 21,419.

In its early history Colusa was one of the most reliable Democratic counties in California.

Colusa County is in California's 1st congressional district, represented by Republican Doug LaMalfa.

[41] In the State Assembly, Colusa County is split between the 3rd and 4th districts, represented by Republican James Gallagher and Democrat Cecilia Aguiar-Curry, respectively.

[43] On November 4, 2008, Colusa County voted 71.6% for Proposition 8 which amended the California Constitution to ban same-sex marriages.

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