The conference included small public high schools with enrollments between 150-550 students in Boone, DeKalb, Lee and Winnebago counties.

By 2000, the Upstate Illini had grown to be the second largest conference in the state of Illinois and included 24 teams and 3 divisions, the East as well as the West - Dakota, East Dubuque, Galena, Lena-Winslow, Orangeville, River Ridge, Stockton and Warren and the South - Forreston, Freeport Aquin, Lanark (Eastland), Milledgeville, Mt.

These divisions were based on geography and not enrollment with the hope of keeping rivalries alive and travel time minimized.

During the spring of the 1999-2000 school year, at the annual conference meeting, the issues of realignment created a sticking point that found the administrators of the East Division at odds with the majority of the league.

The conference ceased in 2005 with North Boone joining the Big Northern to effectively disband the Four Rivers.