Shasta County was one of the original counties of California, created in 1850 at the time of statehood. Parts of the county's territory were given to Siskiyou County in 1852, and to Tehama County in 1856.
According to the U.S. Census Bureau, the county has a total area of 9,965 km² (3,847 mi²). 9,804 km² (3,785 mi²) of it is land and 161 km² (62 mi²) of it is water. The total area is 1.62% water.
As of the census2 of 2000, there are 163,256 people, 63,426 households, and 44,017 families residing in the county. The population density is 17/km² (43/mi²). There are 68,810 housing units at an average density of 7/km² (18/mi²). The racial makeup of the county is 89.32% White, 0.75% Black or African American, 2.77% Native American, 1.87% Asian, 0.11% Pacific Islander, 1.71% from other races, and 3.47% from two or more races. 5.51% of the population are Hispanic or Latino of any race.
There are 63,426 households out of which 31.70% have children under the age of 18 living with them, 53.00% are married couples living together, 11.90% have a female householder with no husband present, and 30.60% are non-families. 24.70% of all households are made up of individuals and 10.20% have someone living alone who is 65 years of age or older. The average household size is 2.52 and the average family size is 2.98.
In the county the population is spread out with 26.10% under the age of 18, 8.20% from 18 to 24, 25.30% from 25 to 44, 25.20% from 45 to 64, and 15.20% who are 65 years of age or older. The median age is 39 years. For every 100 females there are 95.10 males. For every 100 females age 18 and over, there are 91.20 males.
The median income for a household in the county is $34,335, and the median income for a family is $40,491. Males have a median income of $35,959 versus $24,773 for females. The per capita income for the county is $17,738. 15.40% of the population and 11.30% of families are below the poverty line. Out of the total people living in poverty, 21.00% are under the age of 18 and 7.30% are 65 or older.