Manafort's plea deal with Mueller dramatically ramps up the legal and political danger already facing Trump, who is being eyed by the special counsel for possible obstruction of justice.
The deal came three days before the scheduled start of jury selection in U.S. District Court in Washington for Manafort's second trial. It also came several weeks after Trump blasted the practice of "flipping" — prosecutors getting defendants to cooperate against other people by offering them a plea deal — and said the commonly used tactic "almost ought to be illegal."
Prosecutors said the Manafort's own deal, which will require him to forfeit an estimated $46 million in assets, includes a 17-page cooperation agreement with Mueller in which he will help the special counsel cooperate "in any and all matter as to which the government deems the cooperation relevant."
Specificially, Manafort must participate in interviews with investigators, provide documents and testify, if needed.
The guilty plea by Manafort, 69, relates to money earned from consulting work on behalf of pro-Russia politicians in Ukraine and predate his tenure as chairman of Trump's presidential campaign.
However, during his time on the campaign, he participated in a controversial meeting with Trump's son, Donald Trump Jr., and son-in-law, Jared Kushner, with a Russian lawyer who supposedly had negative information about Democratic presidential nominee Hillary Clinton.
Mueller is investigating Russia's interference in the 2016 election and possible collusion by members of Trump's campaign in that effort. Mueller is also looking into whether Trump obstructed justice in an effort to influence the Russia probe.
The special counsel previously obtained guilty pleas to crimes from Trump's ex-national security advisor Michael Flynn, campaign advisor George Papadopoulos and former Trump campaign official Rick Gates, who was a Manafort associate.
Last month, Trump's former personal lawyer, Michael Cohen, pleaded guilty to charges brought by federal prosecutors in New York. Those included tax crimes and a campaign finance charge related to the $130,000 hush-money payment Cohen said he gave to porn star Stormy Daniels at Trump's behest to keep her quiet about an affair she says she had with the president, who denies that tryst occurred.
"This had absolutely nothing to do with the President or his victorious 2016 Presidential campaign," said press secretary Sarah Huckabee Sanders as the plea hearing was still going on. "It is totally unrelated."
Rudy Giuliani, Trump's lawyer for the Russia probe, said: "Once again an investigation has concluded with a plea having nothing to do with President Trump or the Trump campaign."
"The reason: the President did nothing wrong and Paul Manafort will tell the truth," Giuliani said.
Within minutes of Giuliani issuing that statement, another Trump lawyer, Jay Sekulow, sent a "corrected statement" to reporters that said the same thing but omitted the words "and Paul Manafort will tell the truth."
Manafort, wearing a dark suit, white shirt and purple tie, admitted guilt to revised charges of just two counts of conspiracy, which will effectively consolidate and resolve the two criminal cases against him in Washington and in Virginia federal courts.
The two charges are conspiracy against the United States and conspiracy to obstruct justice by witness tampering. The first count relates to a wide array of underlying criminal conduct.
"Is what the prosecution said true?" Judge Amy Berman Jackson asked Manafort.
"I did. It is," Manafort replied.
The judge then asked, "Are you ready to tell me if you plead guilty?"
"I am. I plead guilty," Manafort replied.
After the plea hearing in the E. Barrett Prettyman courthouse, Manafort's defense lawyer Kevin Downing told reporters it was a "tough day for Mr. Manafort, but he's accepted responsibility, and he wanted to make sure that his family was able to remain safe and live a good life."
"He has accepted responsibility, and this is for conduct that dates back many years, and everybody should remember that," Downing said.
No sentencing date was set for Manafort, who faces up to 10 years in prison, but who would likely get much less time behind bars than that. He also faces fines of up to $500,000 when sentenced.
He will not be sentenced until his cooperation with Mueller is completed.
Manafort was convicted last month in the Virginia case of bank fraud and tax crimes, which also was connected to his work in Ukraine.
Jury selection for his second trial in Washington had been scheduled to begin on Monday for charges that included money laundering, failing to register as an agent of a foreign government and witness tampering.
Friday's plea deal substantially winnows down the more than 20 criminal counts that Manafort either still faced or had already been convicted of. Before the deal, he faced the possibility of decades in prison, which given his age would have effectively been a life term.
Manafort has been held without bond in jail since June after being charged with trying to influence witnesses against him. He will remain in jail pending his sentencing.
Trump last month had contrasted his ex-lawyer Cohen's guilty plea, which did not include a cooperation agreement, with Manafort's stance at the time of having gone through his first criminal trial and not cooperating with Mueller.
"I feel very bad for Paul Manafort and his wonderful family," Trump wrote in an Aug. 22 tweet. " 'Justice' took a 12 year old tax case, among other things, applied tremendous pressure on him and, unlike Michael Cohen, he refused to 'break' - make up stories in order to get a 'deal.' Such respect for a brave man!"
Read Mueller's case against Manafort: