Allen H. Weisselberg, a longtime lieutenant to former President Donald J. Trump, has reached an agreement with Manhattan prosecutors to plead guilty to perjury charges on Monday, according to people with knowledge of the matter.

Yet Mr. Weisselberg, who for years has remained steadfastly loyal to Mr. Trump in the face of intense prosecutorial pressure, is not expected to implicate his former boss. That unbroken streak of loyalty has frustrated prosecutors and already once cost him his freedom.

Mr. Weisselberg, 76, is now expected to concede that he lied to investigators from the New York attorney general’s office when they were investigating Mr. Trump for fraud. The attorney general, Letitia James, had accused Mr. Trump of wildly inflating his net worth to obtain favorable loans and other benefits.

That civil case recently ended with a judge imposing a huge financial penalty on the former president — more than $450 million with interest. Mr. Weisselberg, who was also a defendant, was penalized $1 million plus interest and permanently banned from serving in a financial position of any New York company.

A lawyer for Mr. Weisselberg, Seth L. Rosenberg, could not immediately be reached for comment.

Mr. Weisselberg’s plea agreement with the Manhattan district attorney, Alvin L. Bragg, comes weeks before the former president will stand trial on unrelated criminal charges. That case, also brought by Mr. Bragg, stems from a hush-money payment, made on Mr. Trump’s behalf, to a porn star during the 2016 presidential campaign.

The perjury plea marks the latest twist in a tortured legal odyssey for Mr. Weisselberg, who squared off against several law enforcement agencies in both civil and criminal trials. As the long-serving chief financial officer for the Trump family business — the former president’s trusted moneyman — Mr. Weisselberg was considered a linchpin in efforts to implicate Mr. Trump.

Mr. Weisselberg has been rewarded for his loyalty to the family he served for nearly a half century: When he left Mr. Trump’s company last year, he was awarded a $2 million severance package that required him not to cooperate with any law enforcement investigation unless legally required.

He also paid a price. In 2022, he pleaded guilty in a tax fraud case. Although he did not implicate Mr. Trump, he agreed to testify against the former president’s company, the Trump Organization, at its trial on the same charges.

In that case, the company was convicted, and Mr. Weisselberg received a five-month sentence. With good behavior, he served nearly 100 days behind bars at the notorious Rikers Island jail complex.

With this latest plea, Mr. Weisselberg now faces additional time on Rikers.

It is unclear how many charges he faces, and whether they will be misdemeanors or felonies.

Still, Mr. Weisselberg’s plea agreement comes at an inopportune time for the former president, just weeks before he is expected to go to trial on a raft of felony charges accusing him of falsifying business records related to the hush-money deal with the porn star, Stormy Daniels. It is the first criminal prosecution of a former president, and the trial is scheduled to begin with jury selection on March 25.

Mr. Bragg has accused Mr. Trump of orchestrating a cover-up of a potential sex scandal involving Ms. Daniels that could have influenced the outcome of the 2016 election.

A guilty plea on Monday could strengthen Mr. Bragg’s hand heading into the trial, deterring other witnesses in Mr. Trump’s circle from lying on the stand. The perjury charges could also discredit Mr. Weisselberg, who has disputed details of the prosecution’s evidence in the case involving the 2016 election.

Although Mr. Weisselberg has committed no violent offense, prosecutors argue that perjury undermines the broader ends of justice and cannot be ignored.

For his part, Mr. Trump has lashed out at Mr. Bragg, a Democrat, accusing him of persecuting Mr. Weisselberg. And Mr. Trump’s allies have lamented that Mr. Weisselberg will have to again serve time behind bars well into his 70s, and have disputed that he lied in the civil fraud case brought by Ms. James, another Democrat.

Ms. James filed her lawsuit in 2022, and it led to a trial late last year. In February, the judge presiding over the nonjury case sided with the attorney general, concluding that Mr. Trump had manipulated the value of his properties. The judge, Arthur F. Engoron, imposed a sweeping array of punishments, including the more than $450 million judgment.

A focus of the case — and Mr. Weisselberg’s testimony — was Mr. Trump’s triplex apartment in Trump Tower, which is 10,996 square feet, but had been listed for years on his annual financial statements as measuring 30,000 square feet.

During the investigation, in a deposition under oath, Mr. Weisselberg played down his involvement in valuing the triplex.

On the witness stand at the trial, Mr. Weisselberg also claimed that he “never focused” on the unit.

Yet soon after, Forbes magazine, which compiles a list of America’s richest people, published an article citing emails and notes showing that Mr. Weisselberg “played a key role in trying to convince Forbes over the course of several years” of the apartment’s value.

During the recent trial, Justice Engoron concluded that Mr. Weisselberg was not a credible witness, in part because of his severance agreement, which is paid in installments over time, as if to keep Mr. Weisselberg in thrall to the Trump family.

“His testimony in this trial was intentionally evasive, with large gaps of ‘I don’t remember,’” the judge wrote in his decision last month, adding that the severance agreement “renders his testimony highly unreliable.”

“The Trump Organization keeps Weisselberg on a short leash,” the justice wrote. “And it shows.”