Jurist presiding at Trump’s civil trial will serve as judge and jury.

Arthur F. Engoron, who is presiding over Donald J. Trump’s civil fraud trial, is an independent and thoughtful — if somewhat quirky — jurist who has served for 20 years in New York City Civil and State Supreme Court.

The 74-year-old judge, a former cabby with a shock of white hair and a penchant for cracking jokes from the bench, will effectively be judge and jury, deciding the fate of Mr. Trump’s New York businesses, which make up a large portion of his real estate empire.

That’s because the case was brought under a little known but powerful New York state law requiring that the matter be adjudicated at what is known as a bench trial, meaning that no jury will hear the case. The judge not only applies the law, as judges do in jury trials, but also decides the facts, a task that a jury would otherwise perform.

And that means that Justice Engoron, a Democrat, will play a far more prominent and consequential role than a judge would at a jury trial, not just during the proceedings, but in the ultimate outcome — unless he is overturned on appeal.

Last week, before the trial began, Justice Engoron issued a decision that itself could have a devastating impact on Mr. Trump and his family business. He ruled that the former president had consistently committed fraud by inflating the value of his assets by billions of dollars. The ruling could strip him of control of some of his flagship New York properties, including Trump Tower and 40 Wall Street.

Justice Engoron has been overseeing the matter for three years. When the state attorney general, Letitia James, was conducting her civil investigation into Mr. Trump’s business practices, the judge resolved disputes over evidence. Then, after she filed the resulting lawsuit a year ago, the judge began hearing arguments and ruled on pretrial litigation.

While Justice Engoron’s demeanor verged on the jovial in the earlier stages — and he still teases the lawyers and allows himself the occasional comic digression — the proceedings have become increasingly contentious. Last year, he held Mr. Trump in contempt, fining him $110,000, and he unsuccessfully sought to have Justice Engoron taken off the case. Last week, in a social media post, Mr. Trump called the judge “deranged.”

Now, as a result of threats, court security officers pick him up at his home in the morning and drive him to the courthouse, officials said. At the end of his workday, the officers drive him home.

Justice Engoron nonetheless seems to maintain his sense of humor. A fan of pop culture references who also revels in puns, he quoted from the Marx Brothers movie “Duck Soup” in a footnote to underscore his position that some of the defense’s arguments were essentially designed to tell him to not believe his own eyes.

“As Chico Marx, playing Chicolini, says to Margaret Dumont, playing Mrs. Gloria Teasdale,” the judge wrote, “well, who ya gonna believe, me or your own eyes?”

Justice Engoron was appointed to the New York City Civil Court in 2003 and was elected — he ran unopposed — to the State Supreme Court in 2015. Before his time on the bench, he served as a law clerk to a State Supreme Court judge in Manhattan.

The atmosphere in his courtroom is somewhat unusual. Beyond the levity he fosters, he discourages members of the public from standing, as is typical, when he enters. He also gives broad latitude to his principal law clerk, Allison Greenfield, perhaps because he himself has served in that role. Ms. Greenfield serves as something of a martinet, in contrast to the judge’s generally genial demeanor.

But Justice Engoron seems to be losing his patience with Mr. Trump. He has consistently ruled against the former president, and his decision last week had withering words for the defenses put forward by Mr. Trump’s lawyers. He called the conduct of the defendants, who include the president’s two adult sons and the family business, “obstreperous” and their arguments “bogus,” saying they had ignored reality when it suited their business needs.

“In defendants’ world,” he wrote, “rent-regulated apartments are worth the same as unregulated apartments; restricted land is worth the same as unrestricted land; restrictions can evaporate into thin air.”

At a hearing two weeks ago, the judge, addressing one of the former president’s lawyers, pounded his fist in apparent frustration, saying, “You cannot make false statements and use them in business.”