Kelly, 67, is replacing Reince Priebus, who was forced out last week after the spectacular failure of Trump's bid to repeal Obamacare.
Kelly, 67, is replacing Reince Priebus, who was forced out last week after the spectacular failure of Trump's bid to repeal Obamacare and as an ugly in-house feud spilled into the open.
"We just swore in General Kelly -- he will do a spectacular job, I have no doubt, as chief of staff," Trump said after the Oval Office ceremony.
"What he has done in terms of homeland security is record-shattering, if you look at the border, if you look at the tremendous results we've had," Trump said.
Trump accentuated the positive in an early morning tweet: "Highest Stock Market EVER, best economic numbers in years, unemployment lowest in 17 years, wages raising, border secure, S.C.: No WH chaos."
Republicans are hoping Kelly, who served as Homeland Security secretary for the first six months of Trump's presidency, will succeed in imposing discipline on a White House whipsawed by controversy.
The chief of staff traditionally manages the president's schedule and is the highest ranking White House employee, deciding who has access to the US leader.
But many question whether anyone can rein in the mercurial, Twitter-happy Trump, who has appeared to encourage the infighting among various factions vying for influence in his administration.
Under pressure from a widening probe into his campaign's contacts with Russia last year, Trump last week attacked his own attorney general for disloyalty, alarming his conservative base, before turning on Priebus.
In another tweet Monday, Trump hinted that Congress's own health insurance plan should be replaced for its failure to repeal Obamacare, his predecessor's signature reform of the US health care system.
"If Obamacare is hurting people & it is, why shouldn't it hurt the insurance companies and why should Congress not be paying what public pays?"
Since taking office six months ago, Trump's tumultuous administration has seen a succession of negative headlines and brewing scandals.
Fueling the fire, the billionaire Republican has parted with a number of top officials including his national security advisor, deputy national security advisor and FBI director, among others -- an unparalleled turnover for such a young presidency.