The only incorporated city in the county is Loyalton.
As of the census of 2000, there were 3,555 people, 1,520 households, and 986 families residing in the county. The population density was 4 people per square mile (1/km²). There were 2,202 housing units at an average density of 2 per square mile (1/km²). The racial makeup of the county was 94.18% White, 0.20% Black or African American, 1.88% Native American, 0.17% Asian, 0.08% Pacific Islander, 1.04% from other races, and 2.45% from two or more races. 5.99% of the population were Hispanic or Latino of any race. 16.5% were of Irish, 14.4% English, 10.5% German, 9.8% American and 7.5% Italian ancestry according to Census 2000. 95.3% spoke English and 3.4% Spanish as their first language.
There were 1,520 households out of which 27.6% had children under the age of 18 living with them, 53.1% were married couples living together, 7.9% had a female householder with no husband present, and 35.1% were non-families. 29.0% of all households were made up of individuals and 11.5% had someone living alone who was 65 years of age or older. The average household size was 2.32 and the average family size was 2.83.
In the county the population was spread out with 23.3% under the age of 18, 4.8% from 18 to 24, 24.0% from 25 to 44, 30.2% from 45 to 64, and 17.7% who were 65 years of age or older. The median age was 44 years. For every 100 females there were 102.0 males. For every 100 females age 18 and over, there were 97.9 males.
The median income for a household in the county was $35,827, and the median income for a family was $42,756. Males had a median income of $36,121 versus $30,000 for females. The per capita income for the county was $18,815. About 9.0% of families and 11.3% of the population were below the poverty line, including 14.3% of those under age 18 and 2.2% of those age 65 or over.
Public transportation in Sierra County is limited to vans run by senior citizen agencies in Downieville and Loyalton. (The general public may ride on a space-available basis)
Sierraville-Dearwater Field Airport is a general aviation airport located near Sierraville.