Adams County borders Denver County on the north side. It's part of the 700+ square mile Denver Metro Area. Adams County was named for Alva Adams. Alva was governor of the the State of Colorado in 1887-1889, 1897-1899, and again in 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).
John D. "Colonel Jack" Henderson came to Colorado to escape accusations of vote fraud in eastern Kansas in 1859. He was the first recorded homesteader in Adams County. He was previously the editor and owner of the Leavenworth Journal and an outspoken pro-slavery politician. When he came to Colorado he settled on Henderson Island in the South Platte River. There he built a ranch, hotel and trading post and developed a brisk business in selling meat and provisions to the multitudes of gold prospectors flooding into Colorado at the time: the Pikes Peak Gold Rush was on. He stayed for a couple years, then returned to eastern Kansas and signed up to fight in the Union Army against the South. His property today is the site of the Adams County Regional Park and Fairgrounds.
Colorado was originally part of Kansas Territory. When Kansas became a state on January 29, 1861, Colorado became Colorado Territory, officially on February 28, 1861. At the time, Adams County was part of Arapahoe County. In 1901 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 held up until November 15, 1902. That's when Adams County finally designated Brighton as the temporary county seat. At the time, Adams County included the land between today's Sheridan Boulevard and the Kansas state line. That was changed on May 12, 1903 when chunks of Washington and Yuma Counties were removed from the eastern portion of Adams County. It was on November 8, 1904 that Brighton was finally 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.