A New York judge on Friday handed Donald J. Trump a crushing defeat in his civil fraud case, finding the former president liable for conspiring to manipulate his net worth and ordering him to pay a penalty of $355 million that could wipe out his entire stockpile of cash.

The decision by Justice Arthur F. Engoron caps a chaotic, yearslong case in which New York’s attorney general put Mr. Trump’s fantastical claims of wealth on trial. With no jury, the power was in Justice Engoron’s hands alone, and he came down hard: The judge delivered a sweeping array of punishments that threatens the former president’s business empire as he simultaneously contends with four criminal prosecutions and seeks to regain the White House.

Not only did Justice Engoron impose a three-year ban preventing Mr. Trump from serving in top roles at any New York company, including his own, but the judge also applied that punishment to the former president’s adult sons for two years and ordered that they pay more than $4 million each. One of the sons, Eric Trump, is the Trump Organization’s de facto chief executive, and the ruling throws into doubt whether any member of the family can run the business in the near term.