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 as soon as 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 on the witness stand in Mr. Trump’s recent civil fraud trial — but not cooperate against the former president. He might also admit to misleading investigators from the New York attorney general’s office, which brought the fraud case against Mr. Trump.

That civil trial ended with a judge imposing a huge financial penalty on the former president — more than $450 million with interest. The attorney general, Letitia James, had accused Mr. Trump of wildly inflating his net worth to obtain favorable loans and other benefits.

Mr. Weisselberg’s plea agreement with the Manhattan district attorney, Alvin L. Bragg, comes weeks before the former president will stand trial on separate 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, Stormy Daniels.

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 implicating Mr. Trump.