Mayor Laura Prati was reported in serious condition at a nearby hospital after being shot in her office by a man with a pistol, the Italian news agency ANSA reported.
The town's deputy mayor was also injured in the incident.
Police identified the gunman as Giuseppe Pecoraro, a former municipal policeman who had recently been transferred to an administrative position.
"I have settled the scores," Pecoraro reportedly said after the shooting.
Cardano al Campo is near Milan.