The verdict is a public rebuke of the biggest US concert ticket seller, which has been criticised by fans and artists, including Taylor Swift.
The court has not yet determined penalties, but shares of ticket rivals jumped on the prospect they would receive more room to compete.
Live Nation has several legal challenges pending that could undermine the verdict. If it stands, the company would face further court proceedings where states are expected to seek to force a sale of Ticketmaster and seek damages after the jury found Live Nation's conduct resulted in fans being overcharged.
"A jury found what we have long known to be true: Live Nation and Ticketmaster are breaking the law and costing consumers millions of dollars in the process," New York Attorney General Letitia James said in a statement on Wednesday.
Live Nation shares closed down 6.3 per cent following the verdict, while shares of competitors Vivid Seats and StubHub rose 9.3 per cent and 3.5 per cent, respectively.
Live Nation estimates the damages it would owe in the case are less than $US350 million ($A488 million), and said in a statement it believes the ultimate outcome "will not be materially different" than the settlement it entered with the US Department of Justice mid-trial.
The company set aside $US280 million ($A390 million) to settle with the states, it said.
Fans and politicians had intensified calls to examine Live Nation's 2010 acquisition of Ticketmaster, after the company subjected Swift fans to hours-long online queues for tickets to her 2022 Eras tour.
The DOJ and a coalition of states sued the company in 2024. US District Judge Arun Subramanian trimmed some claims from the case before trial.
The jury found on Wednesday that Live Nation holds illegal monopolies in the market for ticketing services to more than 200 major concert venues, and the market for dozens of large concert amphitheatres booked by artists.
The company unlawfully made use of its amphitheatres conditional on artists using its promotion services, the jury found.
The DOJ settled in March in an agreement that requires Ticketmaster to open ticketing to other vendors at 13 amphitheatres and bars it from retaliating against venues that decline to use Ticketmaster.
Some companies and groups in the entertainment industry criticised the deal as not going far enough.
The verdict is a major win for the states, which have signaled they will amp up their antitrust enforcement in cases where the DOJ has pulled back.
The US Federal Trade Commission has filed a separate case against Ticketmaster, alleging deceptive ticket resale practices.
