Chicago erupted three years ago when the city belatedly released a video showing that a white officer had essentially executed a black teenager named Laquan McDonald and that the police and city officials lied about it for months. The public’s outrage drove the police superintendent and county prosecutor from their jobs. Last Friday, 12 jurors convicted the officer, Jason Van Dyke , of second-degree murder after less than eight hours of deliberation.

The candidates who are vying to succeed Mayor Rahm Emanuel — who announced last month, just before the trial began, that he is not seeking a third term — need to know they could end up on the rocks if they ignore what the case teaches about police brutality and corruption. The verdict, the first time in nearly 50 years that a Chicago officer has been convicted of murder in an on-duty shooting, suggests that Cook County residents are no longer inclined to reflexively accept police lies about the use of deadly force.

The next mayor needs to end the city’s complicity with officers’ deceit and demand that police unions agree in contract negotiations to greater accountability and stricter oversight. Officials must commit to enforcing the elements of a consent decree with the state on police reforms whose final details are being hammered out in federal court.

The cover-up that began as Mr. McDonald lay bleeding on the pavement on Oct. 20, 2014, is the subject of a second trial, set to begin next month. Three other officers are charged with lying about the shooting to justify Officer Van Dyke’s actions, claiming Mr. McDonald had menaced officers with a knife.