As for Kelly, until now, he was US Secretary of Homeland Security, nominated by Trump on December 2, 2016.
Priebus' hold on the White House job has always been in question, more so this week after Trump's new communications director badmouthed him to a reporter for the New Yorker.
Trump has repeatedly praised Kelly for his operation of homeland security, including tighter border controls and a travel ban from six Muslim countries that is now the subject of a lawsuit.
Announcing the switch via Twitter, Trump called retired Marine Corps general Kelly a "great American". By ordinary standards, Spicer and Priebus have been far too loyal to Trump for far too long - to their own detriment. "John has also done a spectacular job at Homeland Security", Trump said in a series of tweets.
That tension came to a head when Priebus, the former RNC chairman, objected to the hiring of the hiring of Scaramucci to be the White House's communications director last week.
The announcement comes after a week of conflict between Priebus and Trump's newly appointed White House Communications Director Anthony Scaramucci, who had accused the chief of staff of leaking damaging information about him.
It was not immediately clear if Priebus had been terminated from that position or resigned.
She warned against his appointment, saying "no Washington insider, regardless of who it is, should serve as President Trump's chief of staff". Set aside any substantive policy issues: The Trump White House simply didn't run as a White House must run on a day-to-day basis.
Reince Priebus said "But I'm feeling good about the fact that he's making a change that makes him comfortable with moving forward".
Several years later, when Trump was again considering a presidential run, he consulted Priebus for guidance.
Even so, Trump brought on Priebus as White House chief of staff - a chief aide who essentially serves as the president's right-hand man for running the government.
Kelly has also pushed for support for Trump's signature campaign pledge to build a wall along the southern border, though he acknowledged at his confirmation hearing that "a physical barrier in and of itself will not do the job". The Department of Homeland Security includes agencies that protect the president, respond to disasters, enforce immigration laws, protect the nation's coastlines and secure air travel. "I think things are going well", he said a few days before the election.