51st Illinois General Assembly

The 51st Illinois General Assembly met from 1919 to 1921. John G. Oglesby of Decatur was the Lieutenant Governor of Illinois and thus ex officio President of the Senate. Adam C. Cliffe of Sycamore was President pro tempore of the Senate. David Shanahan of Chicago was the Speaker of the House of Representatives.

Districts
Illinois was divided into 51 districts, each of which elected one Senator and three Representatives. Districts were last reapportioned in 1901 and would not be reapportioned again until 1947.

The counties of each district were as follows:
 * 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 9th, 11th, 13th, 15th, 17th, 19th, 21st, 23rd, 25th, 27th, 29th, and 31st: Parts of Cook
 * 8th: Lake, McHenry, and Boone
 * 10th: Ogle and Winnebago
 * 12th: Stephenson, Jo Daviess, and Carroll
 * 14th: Kane and Kendall
 * 16th: Marshall, Putnam, Livingston, and Woodford
 * 18th: Peoria
 * 20th: Grundy, Kankakee, and Iroquois
 * 22nd: Vermillion and Edgar
 * 24th: Champaign, Piatt, and Moultrie
 * 26th: Ford and McLean
 * 28th: DeWitt, Logan, and Macon
 * 30th: Tazewell, Mason, Menard, Cass, Schuyler, and Brown
 * 32nd: Hancock, McDonough, and Warren
 * 33rd: Henderson, Mercer, and Rock Island
 * 34th: Douglas, Coles, and Clark
 * 35th: Whiteside, Lee, and DeKalb
 * 36th: Adams, Pike, Calhoun, and Scott
 * 37th: Henry, Stark, and Bureau
 * 38th: Greene, Jersey, Macoupin, and Montgomery
 * 39th: LaSalle
 * 40th: Christian, Shelby, Cumberland, and Fayette
 * 41st: DuPage and Will
 * 42nd: Clinton, Marion, Clay, and Effingham
 * 43rd: Knox and Fulton
 * 44th: Jackson, Perry, Washington, Randolph, and Monroe
 * 45th: Morgan and Sangamon
 * 46th: Jefferson, Wayne, Richland, and Jasper
 * 47th: Madison and Bond
 * 48th: Hardin, Gallatin, White, Edwards, Wabash, Lawrence, and Crawford
 * 49th: St. Clair
 * 50th: Hamilton, Saline, Pope, Johnson, and Massac
 * 51st: Franklin, Williamson, Union, Pulaski, and Alexander