Manafort entered his guilty plea to two felony counts during an hour-long hearing in federal court in Washington, D.C. The plea took place three days before he was to face trial on charges related to his lobbying work for Ukraine and alleged witness tampering. He's already been convicted on eight counts of bank and tax fraud crimes in the Virginia trial.
After the hearing, Manafort's attorney told reporters that his client accepted responsibility so that his family could stay safe and lead a good life.
It's unclear how the possible deal might affect Manafort's pursuit of a pardon from President Donald Trump.
The decision by Trump's onetime personal lawyer Michael Cohen to plead guilty last month in a federal investigation in Manhattan particularly angered the president, who denounced him as a "flipper".
Before Manafort's appearance, there was some discussion that he would plead guilty without cooperating and perhaps later receive a presidential pardon. Other counts were dropped.
"The president did nothing wrong", said Rudy Giuliani, Trump's attorney, in a statement. But, Dershowitz said, the significance of his cooperation will depend on what Manafort knows and is willing to share.More news: IPhone XS, XS Max: First Impression
Manafort had other charges dropped but he still looks likely to go to prison, marking a steep fall from grace for a multi-millionaire who was often at Trump's side as the Republican candidate took US politics by storm in 2016.
The move toward a guilty plea is another reversal for Manafort, who has fought vociferously - but unsuccessfully - against Mueller's probe.
When Manafort eventually goes for sentencing, the agreement notes that Mueller will let the court know how much Manafort did or did not cooperate. But the plea nonetheless makes Manafort the latest associate of Trump, a president known to place a premium on loyalty among subordinates, to admit guilt and co-operate with investigators in hopes of leniency.
White House press secretary Sarah Sanders said the Manafort case was unrelated to Trump.
On another conspiracy-related front, as Natasha Bertrand points out today, Manafort might be in a good position to shed light on what exactly former Trump campaign aide George Papadopoulos - who has testified as part of his own plea deal that he learned that Russian Federation had stolen dirt on Clinton in the form of "thousands of emails" - might have told campaign higher ups about this at the time, another big unknown.
The main focus of prosecutors in the second trial was the allegation that Manafort lobbied on behalf of Ukrainian politicians without registering under the Foreign Agents Registration Act.
After his arrest in October, he was detained by the court in his Alexandria, Virginia, home for more than eight months. The jury in Virginia found that Manafort hid millions of dollars he made in Ukraine to avoid paying taxes and then lied to get loans when the political party that was paying him was ousted from power and the funding dried up. Jurors deadlocked on 10 other counts.