The fire was on the third floor of the Cook County side of the building, which is located at 118 N. Clark St. downtown, about 11:15 a.m., CFD said.
There was a substantial amount of smoke on the third and fourth floors, due to the fire involving cardboard boxes in a records storage room, CFD said.
Some fans were brought in to help clear out the smoke, but the fire was quickly put out.
CFD speaks on City Hall fire
Chicago fire crews were looking at two people who suffered smoke inhalation, but they were not taken to a hospital.
One person tripped while evacuating, but also did not go to a hospital, CFD said.
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The Chicago City Council evacuated during their meeting about 11:30 a.m.
City Council evacuates after fire alarm
They resumed about 1:15 p.m.
Chicago Mayor Brandon Johnson's office said he would no longer speak after the meeting due to the "the emergent nature of events and precautions at City Hall."
There was a large emergency response outside the building during the incident.
ABC7 Chicago political reporter Craig Wall was in City Hall at the time of the fire, and said he could smell smoke in the air.
CFD said the accidental ignition of paper products caused the fire.
The Cook County Assessor's Office said in a post on X that their offices will be closed for the rest of the day, after the fire.