The City of Centennial was formed February 7, 2001, from portions of unincorporated Arapahoe County, including the former Castlewood and Southglenn census-designated places (CDPs). The citizens of the area had voted to incorporate on September 12, 2000, choosing Centennial as the official name during the vote. The name reflects Colorado’s admission to the Union as the 38th state in 1876, the centennial year of theUnited States Declaration of Independence. The state of Colorado is nicknamed the “Centennial State”.

Incorporation was approved by 77% of the voters, and the population of the area at over 100,000 made it the largest incorporation in U.S. history as of its creation.