The star witness against Donald J. Trump took the stand on Monday for a fourth and final day at the former president’s criminal trial in Manhattan, fending off a fusillade of attacks from defense lawyers and acknowledging that he once stole from Mr. Trump’s company.

The witness — Michael D. Cohen, Mr. Trump’s onetime personal lawyer and longtime henchman — capped the case for the prosecution, which rested once he left the stand.

Over his week of testimony, Mr. Cohen was the only person to offer firsthand evidence directly linking Mr. Trump to the falsified records that underpin the charges against him. Mr. Trump, he said, approved a plan to fake the records to cover up a sex scandal involving a porn star.

During Monday’s cross-examination, Mr. Trump’s lead lawyer assailed Mr. Cohen’s credibility, painting him as a pathological liar obsessed with taking down the former president. But Mr. Cohen maintained his composure, while some jurors seemed to lose focus as they shifted in their chairs and their eyes wandered. When prosecutors received a second opportunity to question Mr. Cohen, they sought to blunt much of the impact of the cross-examination.

“Are you charged with any crimes in this case?” a prosecutor, Susan Hoffinger, asked him. “No, ma’am,” Mr. Cohen replied, explaining that he was there merely as a “subpoenaed witness.”

However, Mr. Cohen, the 20th and final person to take the stand for the prosecution, was not just any witness. He illustrated much of the prosecution’s case as no one else could, harmonizing disparate facts to portray Mr. Trump as a criminal.

Mr. Cohen took the stand Monday amid a uniquely Trumpian spectacle, as an eclectic entourage of the former president’s supporters — some with legal troubles of their own — packed the courtroom.

The group of more than a dozen on Monday included not only Republican lawmakers and Alan Dershowitz, the high-profile lawyer. There was also a legal adviser to Mr. Trump who is under indictment in Arizona, Boris Epshteyn. There was Bernard Kerik, the former New York police commissioner whom Mr. Trump pardoned for federal felony charges. And then there was Chuck Zito, a former leader of the New York chapter of the Hells Angels motorcycle gang, who spent years in prison on drug charges.

They swept into the courtroom to back up Mr. Trump as his face-off with his former fixer and current nemesis continued.

Now the prosecution will turn over the trial to the defense. It is unclear whether Mr. Trump’s lawyers will mount a case of their own, but they have signaled they may call one of Mr. Cohen’s own former legal advisers to the stand to reinforce their attacks on his credibility.

On Monday, the questioning from Mr. Trump’s lawyer, Todd Blanche, for the first time ventured toward the heart of the case: Mr. Trump’s reimbursement of Mr. Cohen for his hush-money payment to the porn star, Stormy Daniels. Mr. Cohen’s $130,000 payment on the eve of the 2016 presidential election silenced her account of a sexual rendezvous with Mr. Trump that had threatened to derail his campaign.

In return, Mr. Cohen was actually paid $420,000 — an amount that he said included the hush money, a bonus, money for taxes and $50,000 to repay a tech company in an unrelated matter. But when pressed by Mr. Blanche, Mr. Cohen acknowledged that he had pocketed more than half the money earmarked for the tech company, RedFinch.

“You stole from the Trump Organization, is that right?” Mr. Blanche asked.

“Yes, sir,” Mr. Cohen replied.

Mr. Blanche also emphasized how much money Mr. Cohen has reaped from his attacks on his former boss and mentor, Mr. Trump, suggesting that his testimony was motivated by greed, not truth-telling. Mr. Cohen has written two books and is considering a third, and has profited handsomely from a podcast. He has even said he might run for Congress.

And when Mr. Blanche suggested that a conviction would complete Mr. Cohen’s revenge plot, Mr. Cohen corrected him, at least on the economics.

He said it would be better if Mr. Trump escaped unscathed, because “it gives me more to talk about in the future.”

Mr. Blanche sought to finish the pivotal exchange with a flourish, returning to his claim last week that Mr. Cohen had lied on the stand about speaking to Mr. Trump in late October 2016 about the hush-money deal. But Mr. Cohen forced him to end with a whimper, not a bang.

“No doubt?” Mr. Blanche asked Mr. Cohen about his recollection of speaking to Mr. Trump.

“No doubt,” Mr. Cohen replied. And then his cross-examination was done.

There is no way of knowing what the jury thinks of Mr. Cohen, whose past lies and misdeeds were hardly a secret after prosecutors raised them throughout the trial. At this pace, jurors might not begin deliberations until next week: With court in session only three days this week before the Memorial Day holiday, the judge has tentatively set closing arguments for May 28.

On Monday, when Ms. Hoffinger, the prosecutor, had the opportunity to question Mr. Cohen again, she sought to smooth some of the rougher edges of his testimony.

To underscore the idea that Mr. Trump approved of Mr. Cohen’s conduct, she produced a text message from one of Mr. Trump’s lawyers, who expressed appreciation for Mr. Cohen’s telling the media — he now says falsely — that he had paid off Ms. Daniels on his own initiative.

“Client says thank you for what you do,” the text message read, appearing to refer to Mr. Trump.

Ms. Hoffinger also returned to the records that the prosecution says Mr. Trump faked to conceal the hush-money deal. Mr. Trump, who faces probation or up to four years in prison, is charged with 34 felony counts of falsifying business records, one for each document related to his reimbursement of Mr. Cohen in 2017: 11 checks to Mr. Cohen (most of which Mr. Trump signed), 11 invoices submitted by Mr. Cohen and 12 entries in Mr. Trump’s ledger.

The documents all referred to a “retainer agreement,” implying that Mr. Cohen received the money for ordinary expenses. While Mr. Blanche highlighted a variety of legal assignments Mr. Cohen performed for the Trump family around this time, Ms. Hoffinger focused intently on specific sums and records.

“Did the $420,000 that you received in 2017 have anything to do with legal services you provided in 2017?” she asked Mr. Cohen. He bluntly replied: “No.”

“When you submitted each of your 11 invoices,” she then asked, “was that true or false?”

“False,” he confirmed.

And the check stubs that reflected a supposed retainer?

“False,” he told the jury.

Ms. Hoffinger also asked Mr. Cohen to assess the impact of his falling-out with Mr. Trump, who had been the focus of his existence for years.

“My entire life has been turned upside down,” he said.

Maggie Haberman, Kate Christobek and Wesley Parnell contributed reporting.