The 9th congressional district of Illinois covers parts of Cook, Lake, and McHenry counties as of the 2021 redistricting which followed the 2020 census. It includes all or parts of Chicago, Evanston, Glenview, Skokie, Morton Grove, Niles, Northfield, Prospect Heights, Wilmette, Buffalo Grove, Hawthorn Woods, Wauconda, Island Lake, Long Grove, Lake Barrington, Algonquin Township, Cary, Crystal Lake, Lake in the Hills, Lakewood, Oakwood Hills, Trout Valley, Algonquin, Port Barrington, Barrington Hills, and Fox River Grove. It is anchored in Chicago's North Side, along Lake Michigan, and covers many of Chicago's northern suburbs. Democrat Jan Schakowsky has represented the district since January 1999.

Lake County is situated in the northeastern corner of the U.S. state of Illinois, along the shores of Lake Michigan. As of the 2020 census, it has a population of 714,342, making it the third-most populous county in Illinois. Its county seat is Waukegan, the tenth-largest city in Illinois. The county is primarily suburban, with some urban areas and some rural areas. Due to its location, immediately north of Cook County, Lake County is one of the collar counties of the Chicago metropolitan area. Its northern boundary is the Wisconsin state line.