"I am tendering my resignation, honored to have served our nation and the American people in such a distinguished way," Flynn wrote.
Flynn's resignation came hours after news organizations reported that the Justice Department had warned the Trump White House about Flynn's communications with Russia.
The day began with Flynn under siege, but seemingly still hanging on to his job. At 2 pm, he was in the front row at a joint news conference with President Trump and the Canadian Prime Minister.
But several hours later, mixed messages began swirling from the White House.
Democrats immediately seized on Flynn's ouster to demand a broader investigation of ties between Russia and President Trump.
"What did the President know and when did he know it?" said Rep. David Cicilline (D-RI), echoing an infamous line from Watergate.
"Now we need to find out who knew what and when about the Russia connection," said Rep. Jared Polis (D-CO).
Flynn had been controversial from the start, as the former head of the Defense Intelligence Agency - who had been bounced out during the Obama Administration - assumed an outspoken role at Trump campaign rallies, roundly attacking Democrats and Hillary Clinton.
Flynn, and his son - who was ultimately pushed out of the Trump Transition - also trafficked in conspiracy theories about the Clintons on social media before Election Day.
After the elections, Flynn deleted that tweet, and then deleted his Twitter account.
For now, Flynn is being replaced on an acting basis by Retired Lt. General Joseph Keith Kellogg, Jr.
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