Aldermen voted 33-14, falling one vote short of a veto-proof majority, and Johnson said he would use it.
"The illegal dynamic of this ordinance has left me no choice but to veto it," he said after the council session.
With the system set to go silent at midnight Sunday, alderpeople forced an hours-long debate in city council Wednesday.
"It doesn't do the things that ShotSpotter said it would do in the beginning related to higher conviction rates. It just does not do that," said Ald. Andre Vasquez. "Does it provide some level of response? Yes, that's fair."
What happens next in battle over ShotSpotter?
"This is about a gunshot detection strategy so that we know where gunshots are coming from, and we need that," said 3rd Ward Ald. Pat Dowell.
"Now, there are those in this room that don't believe that ShotSpotter is all that it's cracked up to be. They have an issue with something about it. My question to everybody is: What's going to replace it?" 19th Ward Ald. Matt O'Shea said.
There was passionate debate on both sides of the issue, but the vote sent a clear message that a majority of the Council supports keeping ShotSpotter.
"The coalition behind ShotSpotter held. Now the mayor has to recognize that," said Ald. Brian Hopkins.
"I still remain unconvinced that this is an effective and sound and reliable technology," 1st Ward Ald. Daniel La Spata said.
ShotSpotter supporters said the new data from the Chicago Police Department, which was obtained through the Freedom of Information Act, shows the gun detection technology is a valuable life-saving tool.
"If it was good for the DNC, then it's good for us," 37th Ward Ald. Emma Mitts said.
They said the numbers show that from January 2023 to July 2024, more than 1,900 lives were saved because of ShotSpotter.
"If one life is saved with gunshot detection technology, then it is absolutely worth having," 15th Ward Alderman Ray Lopez said. "How can Brandon Johnson, the mayor of the most American of American cities, put a price tag on saving lives?"
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The ordinance voted on today would give the head of the city's Office of Public Safety Administration the right to extend the existing ShotSpotter deal or enter into a new contract for similar technology.
OSA has administrative oversight over the fire, police and emergency communications departments.
Some aldermen wanted to give Chicago Police Superintendent Larry Snelling the option.
"It is a little bit ridiculous to think that you can remove the power from a mayor and then give it to their employee, as if that changes anything. 'Hey, I'm going to give it to the superintendent, who's appointed by the mayor," Vasquez said.
Snelling was at City Hall for another matter, but was unavailable to comment on the ordinance. He has expressed support for ShotSpotter in the past.
"What I believe the superintendent and the mayor will do is get in a room together, compare notes and come up with technology that shares both value systems, that both the superintendent and mayor has," 6th Ward Ald. William Hall said.
The mayor has condemned ShotSpotter, calling it a waste of taxpayer money and "little more than a walkie-talkie on a pole."
"To say that the 91 lives that were saved between January 21, 2023 and July 31, 2024, via alerts from ShotSpotter, no 911 calls, that this is just a walkie-talkie on a stick, is dangerous," 17th Ward Alderman David Moore said.
The ShotSpotter contract expires next week.
"I don't think it's right for us to give this type of contracting authority to the superintendent. I think it's going to cause a lot of problems for us, and what we might have to legally defend," 49th Ward Ald. Maria Hadden said.
Supporters said in a letter ShotSpotter officials promised not to turn off the technology if an ordinance were passed, and would agree to a 15-month extension with a 48% discount on the cost.
"Police work is hard," said Carisa Parker, with the 22nd District police council. "Why in the 21st century would our world-class city not use advanced policing technology to support our first responders?"
Hopkins, chairman of the Public Safety Committee, said he believed that the aldermen would be successful in their effort to keep ShotSpotter but, if not, they are prepared to go to court.
City records actually peg the overall expenditure on ShotSpotter at $53 million so far.
After the meeting, the mayor made his intentions clear.
While the mayor was firm about vetoing the ShotSpotter extension, he said that was not simply the end of the matter.
"I am committed to making sure that we are coming up with a collaborative approach to find systems that will actually work," he said.
Corporation Counsel Mart Richardson Lowry explained Johnson's statement.
"The mayor referenced the veto because the thing that passed is in violation of Separation of Powers Act. Legislative branch cannot compel the executive branch to act. Hence, the item is a vetoable item, and the mayor is going to exercise that privilege," he said.
In a statement Wednesday, SoundThinking, which owns ShotSpotter, said: "We are pleased with the City Council's vote today and are hopeful that we can continue to serve the people of Chicago, specifically in areas plagued by gun violence. We are gratified by the support of the many alderpeople and civic leaders across the City who recognize the undeniable fact that ShotSpotter saves lives."