William Hale Thompson was an American politician who served as mayor of Chicago from 1915 to 1923 and again from 1927 to 1931. Known as "Big Bill", he is the most recent Republican to have served as mayor of Chicago. Historians rank him among the most unethical mayors in American history, mainly for his open alliance with Al Capone. However, others recognize the effectiveness of his political methods and publicity-oriented campaigning, acknowledging him as a "Political Chameleon" and an effective political machine.

The mayor of Chicago is the chief executive of city government in Chicago, Illinois, the third-largest city in the United States. The mayor is responsible for the administration and management of various city departments, submits proposals and recommendations to the Chicago City Council, is active in the enforcement of the city's ordinances, submits the city's annual budget and appoints city officers, department commissioners or directors, and members of city boards and commissions.