[1] Cook moved to Kaskaskia, Illinois in 1815 and took a job as a store clerk, but soon began to read law under the supervision of his uncle, Nathaniel Pope.

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.

[2] 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.

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.

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.