The 1st Illinois General Assembly, consisting of the Illinois Senate and the Illinois House of Representatives, met from October 4, 1818, to March 31, 1819, during the first two years of Shadrach Bond's governorship, at The Kaskaskia State House.

The apportionment of seats in the House of Representatives was based on the provisions of the First Illinois Constitution.

Political parties were not established in the State at the time.

It was succeeded by the 2nd Illinois General Assembly.

Senators and Representatives were both allotted to counties roughly by population and elected at-large within their districts.