The President took to his Twitter handle to announce this.
“I am pleased to announce that Mick Mulvaney, Director of the Office of Management & Budget, will be named Acting White House Chief of Staff, replacing General John Kelly, who has served our Country with distinction. Mick has done an outstanding job while in the Administration….
….I look forward to working with him in this new capacity as we continue to MAKE AMERICA GREAT AGAIN! John will be staying until the end of the year. He is a GREAT PATRIOT and I want to personally thank him for his service!”