Trump and his staff do not wear masks regularly and the president has denigrated people who wear them.
So I just went for a test, and we'll see what happens.
The first lady also tweeted about her diagnosis.
The first lady, who is 50, has a "mild cough and headache", Conley reported, and the remainder of the first family, including the Trumps' son Barron, who lives at the White House, tested negative.
A second televised debate with Biden is scheduled for October 15.
The development is likely to bring to standstill his hectic campaign for the November 3 presidential election. She served previously as White House communications director and as a spokesperson for Trump's 2016 presidential campaign.
Barely two weeks ago, Trump had told the U.N. General Assembly that China, where the virus first emerged late past year, should be held accountable for unleashing "this plague on the world". Several people present at the ceremony have now tested positive for the coronavirus, including Utah Sen.
President Trump and First Lady test positive for COVID-19
Trump's official physician, Sean Conley, said in a statement that the president and his wife "are both well at this time ". The development almost a month ahead of the US Presidential elections could alter the course of the polls dramatically.
Earlier, during a call-in interview with Fox News Channel's Sean Hannity, Trump said: "Whether we quarantine or whether we have it, I don't know".
He also suggested she may have contracted it from well-wishing supporters.
The first lady said in a tweet that she and her husband "are feeling good".
In a statement White House spokesman Judd Deere said: "The president takes the health and safety of himself and everyone who works in support of him and the American people very seriously".
The White House has had previous coronavirus scares. The White House physician said the president is expected to continue carrying out his duties "without disruption" while recovering.
The US has become the worst-hit nation by the virus, with over 200,000 dead and seven million confirmed infections.
"Trump has been trailing behind Biden and he has clearly failed to narrow the gap after the first debate".
On Tuesday Trump attended a presidential debate against Democratic nominee Joe Biden.
At one point, Trump mocked his opponent for his frequent wearing of masks, when questioned on his distrust of their efficiency.
"We're in a phase where the situation is worsening", Health Minister Olivier Veran said. "When needed, I wear masks". Trump and other Republicans and conservatives about Mr. Biden having holed himself up in a bunker. "Everybody knows he's a liar".
He added he was surprised she fell ill, considering she often wears a mask. He held a fund-raising event at a private residence in Minnesota on Wednesday afternoon before holding a Make America Great Again rally in Duluth, Minn.