The shakeup was announced shortly after Air Force One landed at Joint Base Andrews, returning from a speech in Long Island, NY.
Kelly, aged 67 and a native of Boston, stepped down in 2016 from USA southern command, where he was responsible for United States military activities and relationships in Latin America and the Caribbean, including the controversial detention facility at Guantánamo Bay.
As homeland security secretary, Mr Kelly took the lead on some of Mr Trump's most controversial policies, including his executive orders suspending the admission of refugees and temporarily barring visitors from several Muslim-majority nations.
After months of speculation about Priebus' fate, Trump tweeted the decision just as he landed in Washington after an event in NY in which he lavishly praised Kelly's performance at Homeland Security.
"I have been fortunate to have served my country for more than 45 years - first as a Marine and then as the Secretary of the Department of Homeland Security", he said.
How to safely view the solar eclipse
An example of the high-altitude balloon that will be launched during the solar eclipse. "It appears from OR to SC ", he said. An employee views the May 2012 eclipse at Arches National Park through solar glasses, which allow safe viewing of the sun.
Reuters, citing a senior White House official, reported Friday that Trump told Priebus two weeks ago that he would replace him as chief of staff.
Rep. Pete King, who introduced the president at his speech at Suffolk Community College in Brentwood, said praised both Kelly and Priebus. "John Kelly is one of our great stars".
"Reince Priebus has left it all out on the field, for our party and our country", Ryan said. Gen. Kelly has been a star, done an incredible job thus far, respected by everybody.
"I think General Kelly is a brilliant pick", Priebus continued. "John Kelly will do a fantastic job".
Kelly is a retired Marine Corps general and could bring military-style discipline to a White House undercut by insider quarrels.
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It's unclear what illnesses were involved exactly, but they used sperm donated by subjects with various inheritable diseases. Many critics are anxious that the practice could lead to " designer babies " that are engineered with genetic enhancements.
"I would just from that one interview but keep in mind I'm from Alabama", Brooks said.
Asked when Kelly was offered the chief of staff job, she said "They've been talking about it for a while".
A source close to Priebus said the former Republican National Committee chairman turned in his resignation on Thursday night, after Scaramucci's rant against him was published by the New Yorker magazine.
"He's very passionate about the president and the president's agenda, and I think he may have let that get the best of him in that conversation".
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Dianne Feinstein (D-Calif.), the top members of the Judiciary Committee, issued a joint statement about Manafort's subpoena. Investigators are also probing whether Manafort engaged in money laundering, according to The Wall Street Journal .