As of the census of 2000, there were 18,804 people, 6,097 households, and 4,578 families residing in the county. The population density was 16 people per square mile (6/km²). There were 6,774 housing units at an average density of 6 per square mile (2/km²). The racial makeup of the county was 64.29% White, 0.55% Black or African American, 2.33% Native American, 1.21% Asian, 0.39% Pacific Islander, 26.68% from other races, and 4.54% from two or more races. 46.54% of the population were Hispanic or Latino of any race. 8.5% were of German, 5.6% English, 5.5% American and 5.4% Irish ancestry according to Census 2000. 58.7% spoke English and 40.4% Spanish as their first language.
There were 6,097 households out of which 41.4% had children under the age of 18 living with them, 59.6% were married couples living together, 9.6% had a female householder with no husband present, and 24.9% were non-families. 21.5% of all households were made up of individuals and 10.1% had someone living alone who was 65 years of age or older. The average household size was 3.01 and the average family size was 3.51.
In the county the population was spread out with 31.6% under the age of 18, 10.3% from 18 to 24, 26.9% from 25 to 44, 19.8% from 45 to 64, and 11.4% who were 65 years of age or older. The median age was 32 years. For every 100 females there were 103.4 males. For every 100 females age 18 and over, there were 103.8 males.
The median income for a household in the county was $35,062, and the median income for a family was $40,138. Males had a median income of $32,210 versus $21,521 for females. The per capita income for the county was $14,730. About 13.0% of families and 16.1% of the population were below the poverty line, including 19.5% of those under age 18 and 8.2% of those age 65 or over.
Colusa County Transit runs buses from Colusa to Williams, Arbuckle, Grimes and College City, with limited service to Maxwell.
Colusa County Airport is a general-aviation airport located near the city of Colusa.