Any plea agreement must have followed a "proffer", a formal declaration by Manafort or his attorneys about the scope of the information he would be able to provide the prosecutors.
In myriad, secretive and criminal ways, Manafort made tens of millions illegally advancing Kremlin interests, for instance by working with Israelis to smear Ukraine's pro-democracy government as anti-Semitic.
It's unclear how the deal might affect any Manafort pursuit of a pardon from Trump, though Giuliani told Politico before the deal that a plea without a cooperation agreement wouldn't foreclose the possibility of a pardon.
When Paul Manafort was first charged by prosecutors a year ago, court documents read like the opening scene of lives of the rich and famous: From multiple properties complete with gardens manicured in the form of an "M" for the GOP operative's name to high-priced shopping sprees to custom-made suits.
Inside the White House on Friday after the plea, the mood was "oddly calm", said one Republican in frequent touch with officials there.
He originally faced seven charges brought by the special counsel, including obstruction of justice, failure to register as a foreign agent and conspiracy to launder money.
White House press secretary Sarah Huckabee Sanders issued a brief statement after the announcement.
Stone claimed in 2016 to have a "back channel" to WikiLeaks and seemed to predict some of the email dumps that roiled the final stretch of the presidential campaign and damaged Clinton. Manafort attended the infamous June 2016 Trump Tower meeting with Russians, Don Jr. and Jared Kushner created to get "dirt" on Democratic nominee Hillary Clinton.
Trump has denied any collusion with the Russians by his campaign and has long denounced the special counsel's investigation as a political witch-hunt.
U.S. imposes new $200 billion tariffs on China
On June 1, Trump imposed tariffs of 25 per cent on steel and 10 per cent on aluminum from the EU, Canada and Mexico . The trade war between America and China has escalated, fuelling fears that the global economy could be dragged down.
"I think he potentially knows a lot of information, just in light of his role as the campaign chairman during that crucial time during the summer of 2016", said McQuade, who watched much of Manafort's first trial.
President Donald Trump has not directly commented on Paul Manafort's deal on Twitter yet, but his lawyer Rudy Giuliani has been tweeting a lot about it this weekend. He imagined Mueller would tell Manafort the same thing he used to tell his own cooperating witnesses: "We can deal with the truth, whatever the truth is". He also agreed to give up five properties and a handful of financial accounts.
The agreement also calls for Manafort to "cooperate fully, truthfully, completely, and forthrightly" in "any and all matters" the government "deems the cooperation relevant".
But the veteran political strategist has been held in jail for the last several months after prosecutors accused of him witness tampering, and the government has tentatively seized his accounts at three different banks.
So far, the special counsel's office has charged 32 people, and six have pleaded guilty.
The President himself continues to discredit the entire investigation as a "witch hunt" or a plot by the so-called "deep state" - the political-security establishment - in cahoots with the Democratic Party to tarnish his election win and ultimately impeach him. "It shows you've admitted your guilt", he said on Friday before a deal was announced.
The nature of Manafort's deal, however, may mean that the pardon ship has sailed.
Donald Trump's former campaign chair have to say? That meeting is now a focus of the Mueller investigation. Barring a presidential pardon, the 69-year-old was facing a maximum of 80 years in prison.
Mr Trump had praised his stance, suggesting the former aide had been treated worse than Al Capone, the gangster.