In addition, she's also imposing a new curfew for all of Chicago.
The new measures come as the city's business community warns the downtown violence could have disastrous repercussions.
Millennium Park is the number one tourism draw in the Midwest, but it has also increasingly been attracting trouble.
"The scene (Saturday night) devolved into one of chaos and violence," said Lightfoot.
At a City Hall news conference Monday morning, Lightfoot announced the dramatic steps after hundreds of teens – prompted by social media – flooded the park over the weekend. There were fights, thefts and a shooting near ‘The Bean’ that killed 16-year-old Seandell Holliday of the Roseland neighborhood.
A 17-year-old boy has since been charged in that shooting.
After police closed the park, the crowd spilled into the Loop where there were two more shootings and dozens of teens arrested.
"Today I’m signing an executive order to move the curfew back on weekends to 10 p.m.," Lightfoot announced.
The mayor said she was left with no choice.
"It's extremely distressing to learn that pre-teens, as young as ten or eleven, 12-years-old, have been coming downtown by themselves at night," Lightfoot said. "That's simply not smart and it's definitely not safe."
But 15th Ward Alderman and mayoral candidate Raymond Lopez accused the mayor of playing whack-a-mole.
"Clearly we need a strategy and all the mayor is doing today is trying to find cover," said Lopez. "We need a citywide strategy to address this. Because if you squeeze here, crime's gonna go over here."
The downtown crime appears to be having an economic impact. Chicago hotels right now are running at just 57-percent occupancy, the lowest rate of any major American city.
"For us in the hotel industry, we were really excited for this summer and the recovery it was going to help us bring from the pandemic," said Michael Jacobson, president of the Illinois Hotel & Lodging Association. "And I think crime and public safety could almost single handedly undercut the recovery we planned for this summer."
Mayor Lightfoot is also calling on parents to play a bigger role.
"You need to make sure you understand what your young people are doing. Who they are with and where they are going," Lightfoot said.
"Now I want to be clear – it gives me no pleasure to impose these rules and restrictions. But having exhausted every other opportunity, every other tool and remedy – we have to go to this next step, to make sure our jewel of Millennium Park is available and open to everyone," she added.