He claims he worked with Manafort to knowingly falsify tax returns, while also failing to report foreign bank accounts and failing to register as a foreign agent.
Mr Manafort's attorneys signalled they would seek to blame Mr Gates and had accused him of embezzling millions of dollars from Mr Manafort.
Gates testified that Manafort was paid for Ukrainian political consulting work, but that the money was classified as loans instead of income to decrease taxable income. They were the first two people indicted by Mueller's grand jury.
Rick Gates, a longtime business associate of U.S. President Donald Trump's former campaign chairman Paul Manafort, testifies on the fifth day of the trial of Manafort, on bank and tax fraud charges stemming from Special Counsel Robert Mueller's investigation into Russian meddling in the 2016 U.S. presidential election, in federal court in Alexandria, Virginia, U.S., August 6, 2018. With an enormous salary serving his expensive lifestyle (earlier in Manafort's trial, we learned about his taste for ostrich jackets), Manafort had a pretty good racket going on, until he plunged into debt and made the fateful decision to run President Trump's long-shot campaign in 2016.
Manafort, facing charges of bank and tax fraud related to his work in Ukraine, has pleaded not guilty to the charges. Gates also said that they sometimes characterized payments as loans to mask income.
Gates admitted in testimony on Friday that he had lied to Laporta, as well as other accountants and Manafort's bookkeeper.
"Yes", Laporta said, "of course" she would call Manafort if she suspected Gates was enriching himself with Manafort's money.
While the case has nothing to do with either man's work on the Trump campaign and the charges mostly pre-date that time, the U.S. president has shown interest in proceedings.
In the telling of Manafort's legal team, any problems with his financial paperwork were either mistakes or the fault of Gates, who they say was stealing from Manafort and is now cooperating with investigators to cover his tracks.
Testimony from Mr Manafort's tax preparer continued on Monday.
The warning came after multiple reporters rushed out of the courtroom at the same time Monday after it was announced that Gates would testify Monday.
Gates, who also served in a senior role in Trump's presidential campaign, said he repeatedly lied to hide the bank accounts and, at Manafort's direction, would classify money that came in as either a loan or income to reduce Manafort's tax burden.
Gates said he repeatedly lied to hide the bank accounts and, at Manafort's direction, would classify money that came in as either a loan or income to reduce Manafort's tax burden. But Trump has shown interest in the proceedings, tweeting support for Manafort and suggesting he had been treated worse than gangster Al Capone.
U.S. District Judge T.S. Ellis III, who has clashed with prosecutors as he urged them to hurry up presenting their case, has largely allowed the prosecution to question Gates uninterrupted.
All told, prosecutors allege that Manafort failed to report a "significant percentage" of the more than $60 million they say he received from Ukrainians.
On Monday Gates testified that he has met with prosecutors about 20 times. Kerik, on the other hand, isn't putting any weight on Gates' words because of his motivation for testifying.
One accounting trick saved Mr Manafort $US500,000 ($676,907) in taxes, she said.