Duncan moved to Illinois in the year of its statehood, 1818, settling in Brownsville in Jackson County.

Voters then elected him to represent Illinois's at-large congressional district in Congress in 1826, as he defeated abolitionist and multi-term Congressman Daniel Pope Cook.

Jacksonian Democrats were ascendant in Illinois politics, and Cook had fallen out of favor with them when he voted against Andrew Jackson in the contingent election of 1825.

His final break with the Democrats occurred in June 1834, when he voted to recharter the Bank of the United States.

[3] In his mid-term address two years later, Governor Duncan asked the legislature to pass a state Internal Improvements Act, which would authorize the construction of numerous roads, railroads, bridges, river and harbor improvements, and canals across the state.

Attempting to deal with the national economic Panic of 1837 and seeing the costs of internal improvements piling up, Duncan asked the legislature to repeal the program.

This was controversially done in large part by the successful leadership skills of Springfield's representatives, known as the "Long Nine," one of whom was Abraham Lincoln.