Gates, who is cooperating with prosecutors in the trial of Trump's former campaign chairman, testified that Manafort directed him to report overseas income as loans in order to lower his taxable income.
Rick Gates, a former campaign official for U.S. President Donald Trump, departs U.S. District Court after he and Trump's former campaign manager Paul Manafort attended a hearing in the first charges stemming from a special counsel investigation of possible Russian meddling in the 2016 presidential election in Washington, U.S., October 30, 2017.
Prosecutors will turn to Gates for firsthand support of their accusations that Manafort concealed millions of dollars in foreign income and later lied when he applied for bank loans.
Gates, considered a star witness against Manafort, struck a plea deal with prosecutors to cooperate in the case against his former business partner.
Manafort stared directly at Gates as he read aloud the details of his plea agreement, which could see him receive a reduced sentence, at the direction of a prosecution lawyer.
Testimony from Mr Manafort's tax preparer continued on Monday.
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Gates is also admitting to carrying out a vast scheme to hide millions of dollars in foreign bank accounts on Manafort's behalf.
The jury has heard how Manafort made tens of millions of dollars for political work with pro-Russian politicians in Ukraine.
Notably, after urging prosecutors to speed things along as jurors heard from 14 witnesses last week, U.S. District Judge T.S. Ellis III said late on August 3 that Downing could begin his questioning of Laporta on Monday and wouldn't rush him.
Manafort is on trial here facing bank and tax fraud charges.
Gates took the stand for the first time on Monday afternoon.
"Rick Gates had his hand in the cookie jar and couldn't let his boss find out", Manafort defense attorney Thomas Zehnle said during opening arguments.
But Laporta's testimony raised the stakes for Manafort, legal experts said. One accounting trick saved Manafort $500,000 in taxes, she said. Testifying under immunity, she was the first witness to admit she knew accounting manoeuvres Manafort and Gates requested of her were wrong and could be crimes.