As Flynn stood down he admitted to giving "incomplete information" to Vice President Mike Pence regarding phone calls with Russia's ambassador to Washington. His resignation comes three days after Trump said he would look into the allegations.
A US official told CNN Friday
that Flynn discussed US sanctions with Russia before Trump took office, which could be a breach of the law.
Here's how it unfolded.
Trump as President-elect appoints Flynn as his national security adviser
. The appointment raises concerns as the retired Lieutenant General has a history of making controversial anti-Muslim remarks
and is accused of mishandling classified information.
Flynn was fired from his role as the director of Defense Intelligence Agency in 2014 by the Obama administration over claims he was a poor manager.
Sanctions and phone calls
The Obama administration announces new sanctions against Russia
and the expulsion of 35
of its diplomats over the country's alleged interference in the 2016 US election. Flynn and Russian ambassador Sergey Kislyak speak several times
on the phone the same day.
January 12: The Washington Post first reports
that phone calls took place the day the White House announced Russian sanctions.
Pence refutes sanctions talk
Trump's spokesman Sean Spicer confirms Flynn and Kislyak have been in communication, but Vice President-elect Mike Pence tells CBS that the two men did not talk sanctions.
"They did not discuss anything having to do with the United States' decision to expel diplomats or impose censure against Russia," Pence says.
And as questions swirl over the allegations, Trump officials then claim
the communications were focused on scheduling a call between Trump and Russian President Vladimir Putin.
Flynn under investigation
Three days after Trump officially takes office, US officials say investigators are scrutinizing several calls
between Flynn and Russia's ambassador. Spicer, now the White House Press Secretary, reiterates that Flynn told him sanctions were not discussed in the calls.
At some point in January, the Justice Department warns the Trump administration
that Flynn misled administration officials regarding his communications with Kislyak and is potentially vulnerable to blackmail by the Russians, according to a person familiar with the matter.
The message is delivered by Acting Attorney General Sally Yates, who is fired on January 30 for refusing to enforce Trump's controversial travel ban barring citizens from seven Muslim-majority countries from entering the US.
Official: Flynn did talk about sanctions
An aide close to Flynn says he cannot rule out that the adviser spoke about sanctions on the call with Kislyak. On the same day, Trump says he is unaware of reports that Flynn may have spoken about sanctions during the calls and says he will "look into that."
A US official then confirms that Flynn and Kislyak did speak about sanctions
, among other matters.
Russia again denies the allegations that the men discussed sanctions, telling CNN:
"We have already said there haven't been any.
" On the same day, reports surface of the Justice Department's warning to the administration regarding Flynn.
As the reports emerge, Trump's counselor Kellyanne Conway tells MSNBC that Flynn "does enjoy the full confidence of the president," but around an hour later, Spicer says Trump is "evaluating the situation."
Flynn resigns a few hours later
, admitting he had "inadvertently briefed the Vice President-elect and others with incomplete information" regarding the phone calls with Kislyak and apologizes.
He lasted 23 days after being sworn in at a White House ceremony on January 22.