Denver, Colorado Panorama, 1898. Colorado Territory was created on February 28, 1861 and Arapahoe County was formed on November 1, 1861. Denver City was incorporated on November 7, 1861 and served as the Arapahoe County Seat from 1861 until Arapahoe County was split and Denver County created on November 15, 1902. In 1867, Denver City became the Territorial Capital. With its new-found importance, Denver City shortened its name to Denver. On August 1, 1876, Colorado was admitted to the Union.