Territorial governor Ninian Edwards appointed young Cook the territorial Auditor of Public Accounts in 1816, so Cook moved to Edwardsville, Illinois, and purchased The Illinois Herald newspaper from Matthew Duncan, renaming it The Western Intelligencer. Uncle Nathaniel Pope became a delegate to the U.S. Congress from the Illinois Territory, so upon the election of James Monroe as president, Cook moved to Washington, D.C. to establish his career in the nation's capitol. In 1817 Cook travelled to London to deliver dispatches and bring back John Quincy Adams, the country's representative to Great Britain, whom President Monroe appointed to serve as secretary of state. The two men became closely acquainted during the long voyage back. Shortly after Cook returned from England, tired of service as a mere dispatch-bearer, he moved back to Illinois, where he became an ardent supporter of statehood. Cook used his newspaper and new appointment as clerk to the territorial house to influence the Territorial Legislature, which unanimously passed a resolution urging statehood on December 10, 1817. Cook also lobbied his friends back in Washington and Virginia, and his uncle conveyed the territorial resolution to the U.S. Congress on January 16, 1818. After both the U.S. Senate and House agreed, President Monroe on April 18, 1818, signed the law authorizing Illinois to hold a convention to adopt a state constitution and elect officers. On December 3, 1818, President Monroe then signed the law admitting Illinois as the 21st state. Despite his successful advocacy of statehood, Cook was unsuccessful in his first attempt to be elected to the U.S. House of Representatives, losing to John McLean by only 14 votes for the short term remaining after Illinois became a state. However, the new state's legislature appointed Cook as the first attorney general of Illinois. Cook also had briefly served the territory as judge of the western circuit. Again running for Congress in 1818, Cook defeated McLean in the general election, and again in 1820, 1822 and 1824, thus serving as the second representative from Illinois. While in Congress, Cook served on the Committee on Public Lands and later on the Ways and Means Committee. He secured a grant of government lands to aid in the construction of the Illinois and Michigan Canal. In the 1824 election, Cook also helped defeat a proposed convention to legalize slavery in Illinois, and at year's end helped elect John Quincy Adams as President. Cook, in ill-health, campaigned little in 1826, and while he again scored more votes than McLean, the pro-slavery Jacksonian Democrat, Joseph Duncan, won the election. The following spring, President Adams sent Cook on a diplomatic mission to Havana, Cuba, but that did not restore his health. He returned to Edwardsville, and asked to be taken back to his birthplace in Kentucky.
Daniel Cook, who always suffered from poor health, died on October 16, 1827, at the age of 33 in Scott County, Kentucky. Four years after his death, the home county of Chicago, Cook County, Illinois, was named in his honor. He was buried in Kentucky, then reburied with the Lamb family in 1866 in Oak Ridge Cemetery in Springfield, Illinois.
