Inside a staid Manhattan courtroom this week, flashes from a bygone era appeared, recollections of a celebrity-studded world of leveraged secrets and traded favors, and one in which publications sold at supermarket checkout counters wielded real cultural and political power.

It was a world that David Pecker, the first witness called in Donald J. Trump’s criminal trial and the former publisher of The National Enquirer, once presided over. Mr. Trump, his old friend and associate, sat silently at the defense table as Mr. Pecker testified not only about their own dealings, but also about his brushes with other celebrities: Arnold Schwarzenegger. Mark Wahlberg. Tiger Woods.

His testimony, over four days, evoked the sensational, transactional tactics of tabloid newspapers and magazines. But it was also particular to Mr. Pecker, who over two decades ran American Media Inc., the Enquirer’s parent company, commingling journalism and business interests to an extent that other executives had not in his slowly dying industry, according to people familiar with his career.

Once called the “tabloid king,” Mr. Pecker, 72, had been written about often in his decades-long career, but he had never spoken so publicly about how he operated before taking the witness stand.

Under questioning from prosecutors for Alvin L. Bragg, the Manhattan district attorney, Mr. Pecker walked the jury through his role in an effort to suppress negative news about Mr. Trump during the 2016 presidential election. He said he helped orchestrate hush-money deals related to supposed sexual encounters, and an uncorroborated story about an out-of-wedlock child.