McMaster, the head of the Army Capabilities Integration Center, will become one of Trump's top national security and foreign policy advisers, taking the helm of the White House's National Security Council, which was left rudderless after Flynn was forced to resign after just 24 days on the job.
Retired Lt. Gen. Keith Kellogg, who had been serving as the acting national security adviser since Flynn's exit, will return to his role as chief of staff of the National Security Council.
Trump announced his decision Monday seated alongside McMaster and Kellogg at his Mar-a-Lago residence in Florida, where a day earlier he interviewed McMaster and several other candidates for the national security adviser post.
"He is a man of tremendous talent and tremendous experience," Trump said of McMaster. "I watched and read a lot over the last two days. He is highly respected by everybody in the military, and we're very honored to have him."
McMaster called the appointment a privilege and said he looks forward to doing "everything I can to advance and protect the interests of the American people."
Not first choice
McMaster was not Trump's first choice to assume the role of national security adviser. Trump initially offered the position last week to retired Vice Adm. Robert Harward, who turned it down amid concerns about how the White House was being run, sources told CNN last week.
The search for a new national security adviser was sparked by a report that revealed Flynn had discussed sanctions with Russia's ambassador to the US, Sergei Kislyak, before the Trump administration came into office and did not disclose that detail of the conversation to Vice President Mike Pence. Trump said he fired Flynn because of his failure to disclose those details to Pence, but not because of the conversation itself with the Russian ambassador.
McMaster will take on the role after having served several tours of duty in Germany, Southwest Asia and Iraq, including a stint as special assistant to Gen. David Petraeus when he was commander of the US-led coalition forces in Iraq during the 2007 troop surge.
McMaster has a PhD in military history from the University of North Carolina at Chapel Hill. He was tapped for national security adviser over several other finalists for the position, including John Bolton, the hawkish ambassador to the United Nations under George W. Bush who also served in senior positions in the administrations of George H.W. Bush and Ronald Reagan.
Trump signaled Monday that Bolton would soon get a role in his administration "in a somewhat different capacity" despite having been passed over for the national security adviser spot. The President noted that Bolton has "a good number of ideas that, I must tell you, I agree with."
Trump's pick earned quick praise from a contingent of Republican foreign policy leaders in Congress.
California Rep. Devin Nunes, chairman of the House Select Committee on Intelligence, called McMaster "a fine addition" to Trump's national security team and pointed in particular to McMaster's "history of questioning the status quo and infusing fresh thinking and new approaches into military affairs."
Sen. John McCain, the Armed Services Committee chairman who has been among Trump's chief Republican critics on Capitol Hill, called McMaster "an outstanding choice."
"I have had the honor of knowing him for many years, and he is a man of genuine intellect, character, and ability," the Arizona senator said in a statement. "He knows how to succeed. I give President Trump great credit for this decision, as well as his national security cabinet choices. I could not imagine a better, more capable national security team than the one we have right now."