He was facing a second trial Monday on charges related to Ukrainian political consulting work, including failing to register as a foreign agent.
Image copyright Reuters Image caption Paul Manafort seated (L) as his lawyer speaks in court.
The probe has cast a shadow over the Republican's presidency and Manafort's decision to cooperate deals a setback to Trump ahead of congressional elections on November 6. Trump has denied colluding with the Russians and the Russians have denied interfering.
Gates pleaded guilty in February to helping Manafort use bank accounts in Cyprus and Grenadines to hide millions they had made while lobbying for Ukrainian politicians.
Note: This article has been updated with news that Manafort's plea deal is a cooperation agreement. Trump had saluted that stance, publicly praising him and suggesting Manafort had been treated worse than gangster Al Capone. Mueller was authorized to look into matters that arose from the Russian Federation investigation, leading to the charges against Manafort, which are not related to the Trump campaign or to Russian Federation. "Mueller would not have given him a deal unless Manafort was able to help Mueller make a case against someone else or significantly strengthen an existing case".
Mueller's team filed a document today, outlining charges of Manafort money laundering and tax fraud, and attempts to tamper with witnesses to obstruct justice. He will also forfeit assets.
In the new filing, prosecutors dropped the bulk of the charges against Manafort, filing new paperwork that includes just two counts that resemble in many ways the original allegations made in an indictment past year. The president has previously signaled he's sympathetic to Manafort's cause, and his attorney Rudy Giuliani told Politico a plea without a cooperation agreement wouldn't foreclose the possibility of a pardon. His business partner Rick Gates, who testified against him in his trial in Virginia last month, pleaded guilty to two criminal charges, including lying to investigators.
Manafort, 69, earned tens of millions of dollars lobbying for foreign governments and spent that money freely, including on a $15,000 ostrich coat, landscaping and real estate. Giuliani added that "the president did nothing wrong".
Donald Trump Jr, who organised the meeting with Kremlin-linked Russian lawyer Natalia Veselnitskaya and others, initially said it was mainly to discuss a programme on adoptions of Russian children. In addition to the financial and lobbying charges against Manafort, the special counsel's team has said it's investigating allegations he colluded with Russian Federation while working for Trump.
In his Virginia trial last month, Manafort was found guilty on 8 counts (5 tax fraud, 2 bank fraud, and 1 of hiding foreign accounts), with verdicts unable to be reached for the remaining 10 counts. Vanity Fair reports that former Trump lawyer Michael Cohen is talking to investigators: "he has bristled at the feeling that he has taken the fall for a man who has refused to take any responsibility or face any outcome himself".