For decades, Allen Weisselberg operated in the shadows of the Trump Organization, a trusted executive so hidden from public view that one former colleague described him as a “ghost man.”

Now the 71-year-old chief financial officer and loyal confidant of President Trump has been thrust into the spotlight, drawn into legal matters swirling around Mr. Trump.

As treasurer of the Trump Foundation, Mr. Weisselberg was cited in a civil suit filed in June by the New York Attorney General’s office, alleging that the charity engaged in self-dealing, though he wasn’t a defendant and hasn’t been accused of wrongdoing. The foundation is fighting the lawsuit, saying it is without merit.

Mr. Weisselberg has been subpoenaed to testify as a witness before a federal grand jury as part of an investigation into the activities of Mr. Trump’s former personal lawyer Michael Cohen, people familiar with the matter say. Mr. Cohen hasn’t been charged, and has denied wrongdoing.

Mr. Weisselberg also has been linked to hush payments to two women who allegedly had sex with Mr. Trump, deals that are under scrutiny in that investigation. Mr. Trump has denied having sex with the women.