"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: