Pat O'Brien won the 6th ward alderman seat two years ago. Previously, he sat on the Rock Island County Board, and served as the 2nd ward alderman in Moline.
In a news release, Mayor Sangeetha Rayapati called O'Brien a "fierce community advocate" who worked to address poverty, environmental issues, and workers' rights.
"His life of public service will be a lasting reminder to all of us of how to care for our neighbors and our community."