Mueller's 'star witness' testifies against former boss Paul Manafort

Special counsel Robert Mueller’s star witness took the stand on Monday in the trial of former Trump campaign chairman Paul Manafort, conceding to jurors that he aided his former boss in committing crimes.

Rick Gates served as Manafort’s business partner for nearly a decade until 2016, when the two joined the Trump campaign for president. Asked by prosecutors on Monday whom he reported to during that time, Gates answered, “Paul Manafort.”

Gates pleaded guilty in February to charges of conspiracy against the United States and lying to federal authorities. Having initially been charged alongside Manafort, Gates has since cooperated with the special counsel as part of their investigation into Russian meddling during the 2016 campaign, telling the court on Monday that he’s met with Mueller’s team “about twenty” times. He has not yet been sentenced.

As part of his plea agreement, Gates confessed to “knowingly and intentionally” conspiring with Manafort to commit a bevy of bank and securities fraud, as well as act as an unregistered foreign agent in conjunction with Manafort’s work in Ukraine.

During his testimony on Monday, Gates told jurors he and Manafort failed to report over a dozen offshore accounts to the government and admitted to shielding several million dollars over the years, all of which “at Mr. Manafort’s direction,” Gates said.

Seated just twenty feet from Gates in the Alexandria, Virginia, courthouse, Manafort appeared focused on his former business partner as Gates walked jurors through the pair’s laundry list of crimes.

Ahead of Manafort’s trial, Mueller’s prosecutors listed Gates as a potential witness, indicating that he was expected to play a central role in the prosecutor’s case against his former boss.

Attorneys representing Manafort have built their defense argument around Gates, painting him as the true culprit behind their client’s alleged crimes and asserting that Manafort’s only mistake was “placing his trust in the wrong person,” referring to Gates.

“Rick Gates had his hands in the cookie jar and he didn't want his boss to find out," defense attorney Thomas Zehnle told the court in opening statements last week, accusing Gates of being “willing to say anything to save himself.”

Prosecutors surprised Judge T.S. Ellis last Wednesday when Uzo Asonye, a special counsel attorney, suggested that Gates “may not” take the stand. Another special counsel prosecutor, Greg Andres, later clarified that prosecutors had “every intention” of calling Gates to the stand.

Manafort is on trial this week in a federal courthouse in Alexandria, Virginia, facing charges of evading taxes on more than $60 million of income earned working for Ukrainian politicians. The alleged crimes occurred before Manafort’s time on the Trump campaign, and while the special counsel’s mandate is to investigate possible foreign interference in the 2016 elections, Mueller was given latitude to pursue other potential crimes that arose during the course of his investigation.

Manafort has pleaded not guilty to all charges.

ABC News' Katherine Faulders and Lucien Bruggeman contributed reporting