Allen H. Weisselberg, former President Donald J. Trump’s longtime financial gatekeeper, pleaded guilty to felony perjury charges in a Manhattan courtroom on Monday, the latest twist in a tortured legal odyssey.

Yet Mr. Weisselberg, who for years has remained steadfastly loyal to Mr. Trump in the face of intense prosecutorial pressure, did not implicate his former boss. That unbroken streak of loyalty has frustrated prosecutors and now, at the age of 76, will cost Mr. Weisselberg his freedom a second time.

The 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.

Mr. Weisselberg, who was led into the courtroom in handcuffs wearing a blue surgical mask and a dark suit, conceded that in recent years he had lied under oath to the New York attorney general’s office when it was investigating Mr. Trump for fraud. The attorney general, Letitia James, sued Mr. Trump in 2022, accusing him 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 in the case, was penalized $1 million plus interest and permanently banned from serving in a financial position at any New York company.

Although Mr. Weisselberg neither committed violence nor orchestrated an elaborate scheme, Mr. Bragg’s prosecutors argued that perjury undermines the broader ends of justice and cannot be ignored.