Maxwell Little was going door to door to garner support for Williams when the shooting occurred at about 1:45 p.m. in the 6600 block of South Marshfield Avenue in the city's West Englewood neighborhood.
Little was on the sidewalk and streaming on Facebook Live when a man wearing a red mask shot him. The video shuts off just as he was shot in the leg.
No one was in custody Sunday night.
Little drove himself to Little Company of Mary Hospital. He was treated and released.
Williams was also going door to door down the street with his children.
"We were actually out here when they were shooting," the candidate said.
Witnesses heard about five shots. The back window of a nearby vehicle was also shattered by the gunfire.
Williams said the shooting is the type of thing he is campaigning to stop.
"It should have never happened. He is an innocent bystander who is coming out to do something good," Williams said.
Ald. Ray Lopez (15th Ward) issued a statement in response to the shooting, calling it "unacceptable."