He's planning a 12 p.m. news conference Friday to talk about his retirement.
July 24 will be the 75-year-old's last day on the job as 33rd Ward Alderman.
First elected in 1975, Mell is most famous for speaking while standing on his desk during the debate over who should succeed Mayor Harold Washington.
Mell also helped son-in-law Rod Blagojevich move up the political ladder to become Governor.
There's been talk Mell would be succeeded by his daughter, Rep. Deborah Mell.
But Mayor Rahm Emanuel says there will be an open application process for the job.