Arapahoe County was named after the Arapaho Indians and was one of the original 17 counties designated on November 1, 1861 , shortly after the creation of the Colorado Territory . It extended further west and north than its present boundaries, including land that now forms Adams County and southern Washington and Yuma counties. Adams was created out of Arapahoe County 's northern portions in 1901 and its eastern portions were transferred to Washington and Yuma counties in 1903.

Although Denver was Arapahoe County 's original county seat, when the City and County of Denver were created in 1902, Arapahoe County 's capital moved to its present location in Littleton . Two exclaves of Arapahoe County remain entirely surrounded by Denver ; one of them constitutes the City of Glendale .

Today, Arapahoe County is a land of contrast. The County spans 850 square miles, has a population of more than 500,000 and is one of Denver 's fastest growing neighbors. While three-fourths of the County is rural, the western part of the County is largely urban with wonderful communities that have accommodated the influx of latter day settlers. Industrial growth included the Martin Marietta plant. The Denver Technological Center , Centennial Airport and Greenwood Plaza are huge complexes, which house offices of world famous corporations. The eastern end of the county remains largely rural with wheat farms and a few cattle and sheep ranches.