Calaveras County was one of the original counties of California, created in at the time of statehood. Parts of the county's territory were given to Amador County in 1854 and to Alpine County in 1864.

The meaning of the word calaveras is " skulls. This county takes its name from the Calaveras River which was reportedly so designated by Spanish explorer Gabriel Moraga when he found, on the banks of the stream, many skulls of Native Americans who had either died of famine or had been killed in tribal conflicts over hunting and fishing grounds.

According to the U.S. Census Bureau, the county has a total area of 2,685 (1,037 mi²). 2,642 km² (1,020 mi²) of it is land and 44 km² (17 mi²) of it (1.62%) is water. A California Department of Forestry report lists the county's area in acres as 663,000.

As of the census² of 2000, there were 40,554 people, 16,469 households, and 11,742 families residing in the county. The population density was 15/km² (40/mi²). There were 22,946 housing units at an average density of 9/km² (22/mi²). The racial makeup of the county was 91.19% White, 0.75% Black or African American, 1.74% Native American, 0.85% Asian, 0.09% Pacific Islander, 2.07% from other races, and 3.31% from two or more races. 6.82% of the population were of any race. 94.5% spoke English and 4.0% Spanish as their first language.

There were 16,469 households out of which 26.70% had children under the age of 18 living with them, 58.90% were married couples living together, 8.60% had a female householder with no husband present, and 28.70% were non-families. 23.30% of all households were made up of individuals and 10.10% had someone living alone who was 65 years of age or older. The average household size was 2.44 and the average family size was 2.85.

In the county the population was spread out with 22.80% under the age of 18, 5.50% from 18 to 24, 22.40% from 25 to 44, 31.10% from 45 to 64, and 18.20% who were 65 years of age or older. The median age was 45 years. For every 100 females there were 98.50 males. For every 100 females age 18 and over, there were 95.70 males.

The median income for a household in the county was $41,022, and the median income for a family was $47,379. Males had a median income of $41,827 versus $28,108 for females. The for the county was $21,420. About 8.70% of families and 11.80% of the population were below the poverty line, including 15.60% of those under age 18 and 6.20% of those age 65 or over.