Adams County is on the north side of Denver and is one of the counties that are included in the 700+ square mile Denver Metro Area. Adams County was named for Alva Adams, Governor of the the State of Colorado in 1887-1889, 1897-1899, and 1905. Adams County contains 1,192 square miles of land and 6 square miles of water. The county seat is Brighton. Adams County completely surrounds the 53-square-mile Denver International Airport (which land is part of the City and County of Denver).

In 1859, the first recorded settler in Adams County arrived. John D. "Colonel Jack" Henderson came to Colorado to escape accusations of vote fraud in eastern Kansas. He had been the editor and owner of the Leavenworth Journal and was an outspoken pro-slavery politician. But he came to Colorado and settled on Henderson Island in the South Platte River where he built a ranch, hotel and trading post where he did a brisk business in selling meat and provisions to the many gold prospectors who were flooding into Colorado at the time as part of the Pikes Peak Gold Rush. He lasted a couple years before returning to eastern Kansas where he signed up to fight in the Union Army against the South in the Civil War. His property today is the site of the Adams County Regional Park and Fairgrounds.

As Colorado was originally part of Kansas Territory, when Kansas became a state on January 29, 1861, Colorado was broken off and became Colorado Territory, officially on February 28, 1861. At that time, Adams County was part of Arapahoe County. It was in 1901 that the Colorado Legislature voted to split Arapahoe County into 3 parts: the consolidated City and County of Denver, a new Adams County and the remainder to be South Arapahoe County. Final determination of that creation was delayed until November 15, 1902 when Adams County finally was designated with Brighton as the temporary county seat. At that time, Adams County included the land between today's Sheridan Boulevard and the Kansas state line. That was changed on May 12, 1903 when pieces of Washington and Yuma Counties were carved out of the eastern portion of Adams County. It was on November 8, 1904 that Brighton was chosen by the voters as the permanent county seat.

Adams County again lost land in a 1989 election that transferred 53 square miles of land to Denver for the creation of the Denver International Airport. Another chunk of the county was lost in the 2001 creation of the consolidated City and County of Broomfield.