Arapahoe County is the third most populous of the 64 counties of the state of Colorado of the United States. The United States Census Bureau estimated that the county population was 595,546 in 2012, a 4.1% increase since 2010 census. The county seat is Littleton; the most populous city is Aurora. Arapahoe County is part of the Denver–Aurora Metropolitan Statistical Area and the Denver–Aurora–Boulder Combined Statistical Area. Arapahoe County calls itself "Colorado's First County" since its origins predate the Pike's Peak Gold Rush.
On August 25, 1855, the Kansas Territorial Legislature created a huge Arapahoe County to govern the entire western portion of the Territory of Kansas. The county was named for the Arapaho Nation of Native Americans that lived in the region.
In July 1858, gold was discovered along the South Platte River in Arapahoe County (in present day Englewood). This discovery precipitated the Pike's Peak Gold Rush. Many residents of the mining region felt disconnected from the remote territorial governments of Kansas and Nebraska, so they voted to form their own Territory of Jefferson on October 24, 1859. The following month, the Jefferson Territorial Legislature organized 12 counties for the new territory, including a new Arapahoe County. Denver City served as the county seat of Arapahoe County.
The Jefferson Territory never received federal sanction, but on February 28, 1861, U.S. President James Buchanan signed an act organizing the Territory of Colorado. On November 1, 1861, the Colorado General Assembly organized the 17 original counties of Colorado including a new Arapahoe County. Arapahoe County originally stretched from the line of present-day Sheridan Boulevard 160 miles (258 kilometers) east to the Kansas state border, and from the line of present-day County Line Road 30 miles (48 kilometers) north to the Parallel 40° North (168th Avenue). Denver City served as the county seat of Arapahoe County until 1902.
In 1901, the Colorado General Assembly voted to split Arapahoe County into three parts: a new consolidated City and County of Denver, a new Adams County, and the remainder of the Arapahoe County to be renamed South Arapahoe County. A ruling by the Colorado Supreme Court, subsequent legislation, and a referendum delayed the reorganization until November 15, 1902. Governor James Bradley Orman designated Littleton as the temporary county seat of South Arapahoe County. On April 11, 1903, the Colorado General Assembly changed the name of South Arapahoe County back to Arapahoe County. On November 8, 1904, Arapahoe County voters chose Littleton over Englewood by a vote of 1310 to 829 to be the permanent county seat.
According to the 2000 census, the county has a total area of 805.43 square miles (2,086.1 km2), of which 803.14 square miles (2,080.1 km2) (or 99.72%) is land and 2.29 square miles (5.9 km2) (or 0.28%) is water. The county measures 72 miles (116 kilometers) east to west and 4 to 12 miles (6 to 19 kilometers) south to north.
Two exclaves of Arapahoe County are entirely surrounded by the City and County of Denver, the City of Glendale and the Holly Hills neighborhood, a census-designated place.
As of the census of 2000, there were 487,967 people, 190,909 households, and 125,809 families residing in the county. The population density was 608 people per square mile (235/km²). There were 196,835 housing units at an average density of 245 per square mile (95/km²). The racial makeup of the county was 79.93% White, 7.67% Black or African American, 0.66% Native American, 3.95% Asian, 0.12% Pacific Islander, 4.51% from other races, and 3.16% from two or more races. 11.81% of the population were Hispanic or Latino of any race.
There were 190,909 households out of which 34.90% had children under the age of 18 living with them, 51.20% were married couples living together, 10.60% had a female householder with no husband present, and 34.10% were non-families. 27.00% of all households were made up of individuals and 5.90% had someone living alone who was 65 years of age or older. The average household size was 2.53 and the average family size was 3.11.
In the county the population was spread out with 26.70% under the age of 18, 8.60% from 18 to 24, 33.10% from 25 to 44, 23.00% from 45 to 64, and 8.60% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 97.10 males. For every 100 females age 18 and over, there were 94.20 males.
The median income for a household in the county was $53,570, and the median income for a family was $63,875. Males had a median income of $41,601 versus $31,612 for females. The per capita income for the county was $28,147. About 4.20% of families and 5.80% of the population were below the poverty line, including 7.00% of those under age 18 and 5.10% of those age 65 or over.