Manafort is the fifth person linked to Trump to plead guilty to criminal charges.
For months, analysts, experts and political talking heads have speculated Trump could reward Manafort with a presidential pardon for holding out on a plea agreement - where he would be asked all sorts of questions about his dealings with the president on the 2016 campaign - and going to trial.
Prosecutors dropped five other charges that included lobbying violations and money laundering.
The plea deal would allow Manafort to avoid a second trial that had been scheduled to start next week in Washington.
Some legal experts said that if Trump and his advisers knowingly solicited help from Russian Federation, they may have violated campaign finance laws, and a statute that criminalizes conspiracies to impair the functioning of the USA government. After news of Manafort's conviction, Trump tweeted on August 22 that he felt "very badly" for Manafort and his family, adding, "U$3 nlike Michael Cohen, he refused to "break"-make up stories in order to get a 'deal.' Such respect for a courageous man!"
A jury in that earlier trial found Manafort guilty of eight counts of tax evasion, failing to report foreign bank accounts and bank fraud.
Manafort may have tendered his plea for a variety of reasons including the possibility that it would protect his family from financial ruin and the fact that at age 69, the sentences likely to be imposed in both his Virginia and Washington cases could have ensured that he would spend the rest of his life in prison.
A legal source, supportive of the President and familiar with the Manafort case, said the Trump team does not believe Manafort has anything significant on the President to share with the special counsel.
"The government has gotten everything that it could possibly want", one former top U.S. Justice Department Criminal Division attorney said.
For example, the government did not specify how much of a reduction it will seek on Manafort's sentencing if he follows through on his cooperation agreement.
Manafort had refused to cooperate with Mr Mueller even as the Virginia jury convicted him on bank and tax fraud charges.
"He's accepted responsibility", Manafort's lawyer Kevin Downing, said.
Prosecutors had accused him of hiding from United States tax authorities $16 million ($NZ24.4m) he earned as a political consultant in Ukraine to fund an opulent lifestyle and then lying to banks to secure $20m ($NZ30.5m) in loans.
In pleading guilty, Manafort, the former Trump campaign chairman, would admit he broke the law - a concession he refused to make throughout his Alexandria trial and since he was charged almost a year ago in Washington. Manafort pleaded guilty to one count of conspiracy against the United States and one count of conspiracy to obstruct justice for trying to influence the testimony of two witnesses.
It is unclear what information Manafort is prepared to provide to investigators about Trump or that could aid Mueller's investigation into Russian interference in the 2016 presidential election. For now, Manafort has agreed to cough up close to $50 million to the feds in penalties, enough, incidentally, to pay for the Mueller investigation's activities for several years. Stone's communications have been a subject of the Mueller investigation, sources familiar with interviews of other Stone associates have said. He sat straight or leaned his chin on his right hand throughout a lengthy recital of the charges to which he pleaded guilty.