"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," Trump tweeted. "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!"

The president said Kelly, who recently announced plans to leave the White House, will stay through 2018.

"John will be staying until the end of the year," Trump tweeted. "He is a GREAT PATRIOT and I want to personally thank him for his service!"

The president did not say why Mulvaney will serve in an "acting" capacity.

Mulvaney, who served in Congress before joining the Trump administration, also served as acting director of the Consumer Financial Protection Bureau while simultaneously running the White House budget office.