The county was named for the Arapaho Native American tribe, who once lived in the region.

[2] In July 1858, gold was discovered along the South Platte River in Arapahoe County (in present-day Englewood).

On February 28, 1861, Congress passed an act organizing the Territory of Colorado, using present-day borders.

A ruling by the Colorado Supreme Court, subsequent legislation, and a referendum delayed the reorganization until November 15, 1902.

Arapahoe County is home to nine public school districts: Aurora, Bennett, Byers, Cherry Creek, Deer Trail, Englewood, Littleton, Sheridan, and Strasburg.

In the 1924 Colorado gubernatorial election, Republican Clarence Morley, who was openly affiliated with the Ku Klux Klan, won over 66% of the vote in Arapahoe County, a higher percentage than he received in any other county in Colorado.

[15] In addition, the county for many years was represented in congress by Republican Tom Tancredo, a hardcore conservative known for his controversial anti-immigration, anti-Catholic, and nativist political beliefs.

[16][17] As the Denver Metro Area grew in the 1960s and beyond, Arapahoe County became a classic bastion of suburban conservatism throughout the 1980s.

However, heavy urbanization, demographic changes and population increases - such as the rapid diversification of Aurora's population and younger professionals in the southern suburbs - have caused the county to become much more competitive since the 1990s, eventually changing it to more of a Democratic-leaning suburban swing county.

In 2008, the county swung over dramatically to support Barack Obama, who became the first Democrat to carry it since 1964, and only the second since 1936.

It voted for Obama by a similar margin in 2012, and provided much of Hillary Clinton's statewide margin in 2016 as Donald Trump failed to win even 40 percent of the vote in one of the worst showings for a Republican in the county's history, with the Democrats carrying the former Tech Center area Republican strongholds of Centennial and Littleton.