The CSL traces its history back to the founding of the Suburban League in 1913, of which Evanston and New Trier were both founding members. The league had somewhat fluid membership over several decades. As the population of suburban Chicago grew (and the number of high schools with it) schools began leaving to join newer conferences such as the West Suburban Conference (started 1922), the Northwest Suburban (started in 1925, which later became the North Suburban Conference and Mid-Suburban League), and the South Suburban League (started in 1950). With more schools leaving (and newer suburban schools joining more geographically logical conferences), the Central Suburban League was founded in 1965, the last major conference to be carved from the old Suburban League. The last remaining bastion of the Suburban League started joining these other leagues, and finally folded after the 1974–1975 school year.