Arnold is named after Bob and Bernice Arnold, who, in 1927 opened the Ebbetts Pass Inn. Prior to that, the community was comprised of two large ranches where logging was the main industry. The inn served as a stop for people traveling along the Ebbetts Pass route as well as lodging for those visiting nearby Calaveras Big Trees State Park. In 1928, Camp Wolfeboro was established nearby as a Boy Scout camp and continues to be in operation today.
As of the censusGR2 of 2000, there were 4,218 people, 1,864 households, and 1,325 families residing in the CDP. The population density was 109.9/km² (284.5/mi²). There were 4,456 housing units at an average density of 116.1/km² (300.6/mi²). The racial makeup of the CDP was 95.12% White, 0.24% Black or African American, 0.95% Native American, 0.50% Asian, 0.12% Pacific Islander, 0.71% from other races, and 2.37% from two or more races. 3.34% of the population were Hispanic or Latino of any race.
There were 1,864 households out of which 21.7% had children under the age of 18 living with them, 61.5% were married couples living together, 6.7% had a female householder with no husband present, and 28.9% were non-families. 23.8% of all households were made up of individuals and 9.3% had someone living alone who was 65 years of age or older. The average household size was 2.26 and the average family size was 2.65.
In the CDP the population was spread out with 18.8% under the age of 18, 4.5% from 18 to 24, 19.1% from 25 to 44, 36.6% from 45 to 64, and 20.9% who were 65 years of age or older. The median age was 50 years. For every 100 females there were 103.0 males. For every 100 females age 18 and over, there were 97.2 males.
The median income for a household in the CDP was $42,785, and the median income for a family was $49,364. Males had a median income of $42,941 versus $22,344 for females. The per capita income for the CDP was $23,169. About 7.6% of families and 10.8% of the population were below the poverty line, including 11.9% of those under age 18 and 5.6% of those age 65 or over.