The 1927 Chicago mayoral election was held on April 5. Democratic incumbent William Emmett Dever was defeated by Republican candidate William Hale Thompson, who had served as mayor from 1915 to 1923. Former health commissioner John Dill Robertson, who had been allied with the ex-mayor, broke with Thompson to run on his own and received more than five percent of the vote. It remains as of 2019 the last Chicago mayoral election not won by a Democrat.