During closing arguments at Donald J. Trump’s civil fraud trial, Arthur F. Engoron, the judge who has overseen the case for more than three years, made what might have been an unusual comment for any other jurist.

Justice Engoron, a lean 74-year-old with an unruly mop of white hair, acknowledged that his control of the courtroom had not been perfect.

He had allowed repetitive objections from Mr. Trump’s lawyers, despite protests by the New York attorney general’s office, which brought the case. He had often ignored Mr. Trump’s violations of courtroom decorum. At one point, the judge recalled, he had even let a witness answer his mobile phone while on the stand.

Despite all that, he warned the lawyers, “I don’t want you to think I’m a pushover.”

No one is likely to think so now. Justice Engoron on Friday ruled against the former president, finding that he had orchestrated a conspiracy to inflate his net worth, penalizing him $355 million and instituting a three-year ban from running his family business. Despite his absurdist humor and good cheer, the judge showed himself in the end to be a very serious man.

It was the culmination of what was surely one of the more intense periods of Justice Engoron’s professional life. During the trial, he contended with repeated anonymous antisemitic attacks on his family and on his law clerk, Allison Greenfield, and with threats to his own life. Last month, he was roused early one morning to find that a bomb squad had been dispatched to his Long Island home to respond to a report that turned out to be a hoax.

But despite the attacks, and his own clear desire for harmonious proceedings, Justice Engoron consistently came down hard on the former president. On Friday, he continued his streak of lacerating rulings.

Justice Engoron was an unlikely antagonist for the former president, who has repeatedly denounced him as a Democratic stooge. He is a former cabdriver and music instructor who served in the New York judiciary for more than 15 years before presiding over Mr. Trump’s bench trial. He served as both judge and jury, which the statute under which the lawsuit was brought requires.

He first began overseeing the case in 2020, and over the months, a dualistic approach emerged: He was solicitous in the courtroom, and scathing in written decisions.

In February 2022, for example, he supervised an explosive hearing at which he and Ms. Greenfield focused on keeping the peace. They listened with equanimity as lawyers for Mr. Trump protested that their client should not be subject to questioning under oath.

Then, in his written ruling, Justice Engoron did not hold back, asserting that the attorney general, Letitia James, had found “copious evidence of possible financial fraud” — evidence that he wrote justified the questioning. A couple of months later, he held Mr. Trump in contempt of court for failing to fully respond to a subpoena, eventually penalizing him $110,000.

Justice Engoron moved directly into the spotlight last September, the week before the civil fraud trial started. In a pretrial ruling, he delivered a devastating blow to Mr. Trump, finding that his annual financial statements — which contained representations of his net worth — were filled with fraud.

On the morning of opening arguments, Mr. Trump entered the courtroom ready for a fight, telling the reporters in the hallway that he’d soon be on trial in front of a “rogue” judge. Minutes later, Justice Engoron took the bench and, with Mr. Trump seated before him, seemed wholly unfazed: One of his first comments was a joke about the pronunciation of his own name.

“I’m Judge Arthur Engoron, and that is the correct pronunciation of my surname,” he said. “En-GOR-on, not EN-go-ron or, even worse, En-GU-ron.”

He looked amused as photographers captured Mr. Trump scowling at the defense table, and he posed when they turned their lenses on him. Hours later, videos of Justice Engoron smiling at the cameras set to the theme song of the sitcom “Full House” would be played thousands of times on TikTok.

His lighthearted demeanor persisted. He joked as the photographers snapped similar pictures of Mr. Trump each day, commenting, “You look the same.” He said that he wanted history books to mention his vigor as he bounded up the stairs to the bench. He bantered with his court staff and offered lawyers well wishes on their birthdays.

The judge’s tone lent itself to a permissive atmosphere, and Mr. Trump and his legal team quickly capitalized. They routinely delivered long speeches lamenting the unfairness of the proceedings. They persuaded Justice Engoron to allow hours of testimony from expert witnesses, over the protestations of the attorney general’s office. Justice Engoron even permitted Donald Trump Jr. to deliver a glowing slide presentation about his family’s real estate holdings.

“Let him go ahead and talk about how great the Trump Organization is,” Justice Engoron said.

More than two hours later, the eldest Trump son had discussed his family’s history in real estate dating to the Yukon gold rush, along with the interiors of Trump Tower and his father’s love of golf. As the attorney general’s lawyers bristled, Justice Engoron occasionally smiled at the witness.

Justice Engoron’s tendency to give a green light was not simply the whim of an unconventional legal mind. He explained early in the trial that he wished to avoid a retrial of the case, or any second-guessing of his decisions from an appeals court.

“To me, that basically speaks in favor of allowing, rather than disallowing, the questions, answers, expert testimony, et cetera,” he said.

The judge had only a few red lines. He was infuriated by attacks on Ms. Greenfield, his principal law clerk. Her unusually visible role in the case — she sits next to Justice Engoron on the bench and confers with him on legal matters — drew harsh criticism from the defense.

During the first week of the trial, Justice Engoron issued a limited gag order prohibiting Mr. Trump from commenting on his staff after the former president shared a picture of Ms. Greenfield with Senator Chuck Schumer on Truth Social, calling her “Schumer’s Girlfriend.” Weeks later, the judge fined Mr. Trump $5,000 after learning that a copy of the post was still visible on Mr. Trump’s campaign website. At the time, he threatened steeper fines and possible imprisonment.

But the comments about Ms. Greenfield kept coming. In one of the trial’s most striking moments, Justice Engoron called Mr. Trump to the witness stand and questioned him about his statement to reporters referring to “a person who’s very partisan sitting alongside” the judge, “perhaps more partisan than he is.”

Mr. Trump argued that he had been talking about someone else. But Justice Engoron found that Mr. Trump’s answers were not credible and fined him an additional $10,000 for again attacking the law clerk.

“I am very protective of my staff,” Justice Engoron said that day. “I don’t want anybody killed.”

Behind the scenes, the judge was inundated with hundreds of threats from Mr. Trump’s supporters. They escalated whenever Mr. Trump personally targeted Justice Engoron and Ms. Greenfield, court officials said, requiring constant re-evaluation of the court’s security protocols. Mr. Trump falsely accused the judge’s wife of sharing anti-Trump rhetoric on social media and his son of receiving preferential access to the courtroom.

The disorder in the courtroom and the clamor outside of it seemed to have little impact on Justice Engoron’s view of the case and the fundamental issue at trial: Mr. Trump’s liability.

At one point, the judge denied the defense’s motion to end the case on the spot after bankers testified that they had been satisfied with Mr. Trump as a client. “The mere fact that the lenders were happy doesn’t mean that the statute wasn’t violated,” Justice Engoron said.

On an another occasion that Mr. Trump’s lawyers asked him to throw out the case, Justice Engoron was unyielding.

“No way, no how is this case being dismissed,” he said. “There is enough evidence in this case to fill this courtroom.”

William K. Rashbaum contributed reporting.