Colusa County is located in the Central Valley in California, northwest of state capital Sacramento. The county seat is Colusa. Colusa County is one of the original counties of California, created in 1850 and named after two Mexican land grants; Coluses (1844) and Colus (1845). The county has a total area of 1,156 mi² and 99.5% of it is land and 0.5% of it is water. As of 2000, there were 18,804 people, 6,097 households, and 4,578 families residing in the county. The population density was 16/mi².
The median age was 32 years. For every 100 females there were 103.40 males. The median income for a household in the county was $35,062.