65 ILCS 5/3.1-15-15

A mayor, president, alderperson, trustee, clerk, or treasurer shall not hold any other office under the municipal government during the term of that office, except when the officer is granted a leave of absence from that office or except as otherwise provided in Sections 3.1-10-50, 3.1-35-135, and 8-2-9.1. Moreover, an officer may serve as a volunteer fireman and receive compensation for that service.