A grinning Luigi Mangione appeared to yuk it up in a Manhattan courtroom Monday as he pleaded not guilty to first-degree murder of UnitedHealthcare CEO Brian Thompson.

Mangione, 26, walked into Manhattan Supreme Court at 9:26 a.m. with his feet chained, wearing a maroon sweater over a white collared shirt, tan khakis, and orange prison shoes – an outfit that partly, and oddly, matched the attire of his attorney Karen Friedman Agnifilo.

At least two dozen women packed the courtroom for the twisted heartthrob, with just six men watching on.

“How do you plead to this indictment, sir? Guilty or not guilty,” asked a court clerk.

“Not guilty,” Mangione replied.

During the hearing, Mangione flashed a grin and looked around the court with furrowed brows and a smoldering expression.

Agnifilo, argued before Judge Carro and Assistant District Attorney Joel Seidemann that she was “very concerned” about her “clients’ right to a fair trial.”

“He’s being prejudiced by some statements made by government officials,” Agnifilo said.

“He’s a young man and he’s being treated like a human ping pong ball.”

Agnifilo, who arrived at 100 Centre Street at 8:30 am, beside her husband Marc Agnifilo, then slammed Mayor Adams and the NYPD for Mangione’s perp walk last week — calling Adam’s appearance “unconstitutional.”

“The mayor should know more than anyone about the presumption of innocence… he was trying to detract from those issues,” she said.

What we know about the shooting of UnitedHealthcare CEO Brian Thompson

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• Brian Thompson, the CEO of insurance giant UnitedHealthcare, was gunned down Wednesday outside a luxury Midtown hotel in a “brazen, targeted attack,” police said.
• Thompson was named CEO of UnitedHealth in April 2021. He joined the company in 2004. He was one of several senior executives at the company under investigation by the Department of Justice.
• Thompson’s wife, Paulette, said her husband had been getting threats before he was killed.
• Thompson’s shooting led to sick support online, and even spurred a tasteless lookalike competition in NYC.
• A person of interest has been nabbed by police officers inside a McDonald’s in Altoona, Pa.
• The suspect has been identified as Luigi Mangione, 26, originally from Towson, Md. He’s an Ivy League graduate who hated the medical community.

Follow along with The Post’s live updates on the news surrounding Brian Thompson’s murder.

The dramatic perp walk came after Mangione arrived in the Big Apple last week by helicopter after a brief extradition hearing in Pennsylvania.

He had been expected to be arraigned in state court after his indictment on 11 counts, including first-degree murder in furtherance of terrorism.

But, to the surprise of his lawyers, the Ivy Leaguer was carted to a Manhattan federal courtroom for arraignment on a parallel leveled by the feds with the Southern District of New York.

The surprise federal case came after health insurance industry leaders leaned on the Department of Justice to take up the case against Mangione as a deterrent against copycat killers, sources have told The Post.

Mangione could face the death penalty in the federal case.

The alleged assassin will remain in the federal Metropolitan Detention Center in Brooklyn even as the Southern District prosecutors said they’ll let local DAs get first crack at Mangione in the courtroom, said Seidemann, the assistant district attorney.

“They are allowing us to try our case first,” Seidemann told the court.

As Mangione exited the courtroom, one woman supporter shyly flashed him a thumbs up. Outside, more than 30 supporters of Mangione chanted and held signs in support of the accused killer.

The suspect fled the five boroughs after allegedly shooting Thompson, 50, with a ghost gun Dec. 4 outside the Hilton hotel in Midtown before taking off on a bicycle and hopping a train out of dodge.

After a five-day nationwide manhunt, Mangione was busted in Altoona, Pennsylvania, where he stopped at a local McDonald’s after getting off a Greyhound bus that was passing through town.

An Ivy League graduate from a wealthy Baltimore family, authorities said Mangione planned the murder months in advance over a gripe with the “parasitic” health insurance industry.

Since then, the accused killer has received sick support from people angered at the healthcare industry.