Mr Mangione appeared in court on Monday wearing a maroon sweater, white-collared shirt and khaki trousers.
In addition to a long stream of journalists waiting for the suspect to appear, members of the public – almost all of them young women – were in court, some of whom told CBS, the BBC’s US partner, that they were there to show their support.
Mr Mangione is facing 11 state criminal counts in New York, including first-degree murder and murder as a crime of terrorism.
If convicted of all the counts, he would face a mandatory sentence of life in prison without the possibility of parole.
Federal prosecutors have also separately charged Mr Mangione for using a firearm to commit murder and interstate stalking resulting in death. Both charges could make him eligible for the death penalty.
He has yet to enter a plea on those charges.
Prosecutors have said the federal and state cases will move forward parallel with one another.
In court last week, Mr Mangione’s lawyer – Karen Friedman Agnifilo – said that the two sets of charges appear to conflict, with the state charges accusing him of seeking to “intimidate or coerce a civilian population” while the federal charges focus on crimes against an individual.
Ms Agnifilo said that the overlapping cases were “confusing” and “highly unusual”.
“I’ve never seen anything like what is happening here” in 30 years of practising law, she said.
In court on Monday, she further told the judge that she believed that statements from government officials – including New York City Mayor Eric Adams – make her “very concerned about my client’s right to a fair trial”.
“This is a young man,” she said. “He is being treated like a human ping-pong ball between two warring jurisdictions here.”
She also accused state and federal authorities of treating Mr Mangione like “political fodder” and a “spectacle” by bringing him back to New York by helicopter, surrounded by officials and armed guards, in full view of cameras and journalists.
The judge, Gregory Carro, said that he is unable to control what happens outside court, but vowed that Mr Mangione would receive a fair trial.
In response, the state’s prosecutor said that he had never seen a case with a “higher volume” of quality evidence.