If Kelly does resign from his post, Trump will need to nominate a replacement, who will have to be confirmed by the Senate.
Priebus and Scaramucci flew together on Air Force One to the NY event, but the White House press pool reported they didn't see the communications director on the plane for the return trip.
The abrupt announcement came as Trump landed aboard Air Force One at Andrews Air Force Base in Maryland after a trip to Long Island, New York.
Trump wrote in a tweet: 'I am pleased to inform you that I have just named General/Secretary John F Kelly as White House Chief of Staff. "He is a Great American.", Trump tweeted to his more than 34 million followers, ". and a Great Leader. "He must now quickly replace Secretary Kelly with someone experienced and measured who understands that homeland security is not a partisan issue". "He has been a true star of my Administration", Trump said.
Kelly enjoyed bipartisan support, and was credited in his tenure as secretary of Homeland Security for reducing illegal border crossings and enforcing travel bans targeting citizens of six Muslim countries.
Before Spicer and Priebus, former National Security Advisor Michael Flynn resigned in February over allegations of improper contacts with Russian Federation, and former White House Communications Director Mike Dubke resigned in May.
Reince Priebus said "But I'm feeling good about the fact that he's making a change that makes him comfortable with moving forward. We accomplished a lot together and I am proud of him!" he said.
Shortly before the president deplaned, Priebus' black SUV pulled away, leaving the rest of the motorcade, including the president's vehicle, in the distance.
Finnish president comments on new United States sanctions against Russian Federation
The pipeline under discussion is supposed to carry Russian produced natural gas to the European continent through the Baltic Sea. Russian President Vladimir Putin will visit neighboring Finland on July 27 on a one-day trip.
Priebus the latest official to leave Trump administration
He defended the orders to reporters and lawmakers and insisted that he indeed had been part of the decision-making process. I am pleased to inform you that I have just named General/Secretary John F Kelly as White House Chief of Staff.
USA economy bounces back from weak first quarter
The growth rate also marks an encouraging improvement over Q1's pace, which was revised down slightly to a sluggish 1.2% rise. Average GDP growth during the period through the end of previous year is now at 2.2 percent, up from 2.1 percent previously.
The final go-ahead came during a meeting Thursday night in which Trump finalized that Kelly was his pick to replace Priebus, according to two people familiar with the discussion The president had considered others, including political operatives David Urban and Wayne Berman, for the job.
On Wednesday, Scaramucci went on an expletive-laden rant to a reporter in which he called Priebus a "paranoid schizophrenic, a paranoiac". Priebus had tried unsuccessfully to block Scaramucci's hiring. "I am honored to be asked to serve as the chief of staff to the president of the United States", the now-outgoing Homeland Security chief said in a statement. But what remains to be seen is whether Trump will listen to him as Kelly seeks to bring order to a White House beset by chaos.
Priebus later went on CNN's The Situation Room with Wolf Blitzer, claiming he had resigned from the post. "Reince Priebus is a good man", Trump repeated.
Kelly told Meet The Press earlier this year intelligence leaks were "darn close to treason".
Kelly's departure will leave an immediate vacancy at the department.
Michael Dubke - Founder of Crossroads Media, resigned as White House Communications director in May.
You've heard the old saw - don't do more than one of changing house, job or partner at the same time.
Republican Sen. Lindsey Graham of SC, a frequent Trump critic, welcomed Kelly as "one of the strongest and most natural leaders I've ever known".