The decision in the case, which Mr. Trump’s campaign filed in August 2018 with the American Arbitration Association in New York, comes as the former president is enmeshed in a number of investigations and legal cases related to his private company.

“Donald has used this type of vexatious litigation to intimidate, harass and bully for years,” Ms. Manigault Newman said in a statement. “Finally the bully has met his match!”

The decision, dated on Friday and handed down on Monday, calls for her to collect legal fees from the Trump campaign.

Mr. Trump’s campaign filed the case shortly after Ms. Manigault Newman published her book, “Unhinged.” It claimed that she violated a nondisclosure agreement she had signed during the 2016 campaign stipulating that she would not reveal private or confidential information about his family, business or personal life.