Gates pleaded guilty to one count of conspiracy against the United States and one count of making a false statement. Both counts were issued in a new superseding information on Friday.
According to the new information, Gates knowingly lied when he told the special counsel that Manafort, after a March 2013 meeting with a lobbyist and a member of Congress, had said that Ukraine had not been mentioned in the meeting. Not only was that statement never made, the information says, but Gates himself helped prepare a report describing the discussions on Ukraine that had taken place at that meeting.
That false statement was made on Feb. 1 — the same day Gates' legal counsel filed a motion to withdraw as his defense.
Gates also pleaded guilty to conspiring to defraud the U.S. by making false statements regarding his status as a foreign agent, a charge initially proffered in the first indictment from Oct. 27, 2017.
According to the sentencing guidelines, Gates could face nearly six years in prison. But the special counsel could petition the court to reduce his sentencing time, Reuters reported, based on his cooperation with investigators.
A sentencing date has not been publicly set, although the court tentatively scheduled a status update for May 14.
Gates, 45, had initially pleaded not guilty to charges including money laundering and misleading investigators. A second indictment from the special counsel on Thursday piled 32 new counts on Gates and Manafort, including bank fraud.
In a statement following Gates' change of plea, Manafort said, "I continue to maintain my innocence."
"I had hoped and expected my business colleague would have had the strength to continue the battle to prove our innocence. For reasons yet to surface he chose to do otherwise. This does not alter my commitment to defend myself against the untrue piled up charges contained in the indictments against me," Manafort said.
But Gates' new status as a cooperating witness for the Mueller investigation could increase the pressure on Manafort to seek a plea deal.
"Gates had a front-row perspective for everything Manafort was up to," said Jens Ohlin, vice dean and professor of law at Cornell Law School. "His cooperation was pretty crucial for Mueller."
While Manafort held a more prominent role in Donald Trump's presidential campaign, Gates maintained his connection to it for much longer.
Gates remained a part of the campaign after Manafort resigned as chairman in August 2016. He served as an intermediary between the Republican National Committee and the campaign, and he was hired after the election to help with the inauguration.
In October 2017, Gates and Manafort, 68, were charged by the special counsel on the same day investigators revealed that George Papadopoulos, a foreign policy aide to the Trump campaign, had pleaded guilty to lying to the FBI and was cooperating with the Russia probe.
The New York Times previously reported that Papadopoulos' conversations in 2016 with an Australian diplomat, in which he claimed that Russia had "dirt" on then-presidential candidate Hillary Clinton, formed the nexus of the investigation into potential collusion between the Trump campaign and Russia.
Manafort was indicted on 12 counts in October, including laundering foreign money from foreign sources — notably from his work for political parties supportive of then-Ukrainian President Viktor Yanukovich — to fund an opulent lifestyle.
Gates, Manafort's longtime business associate, was charged on nine of the counts.
Manafort was initially saddled with a $10 million appearance bond, and Gates received a $5 million bond.
In a court document on Feb. 16, which questioned the value of properties Manafort was offering in lieu of his $10 million bail payment, the special counsel announced the discovery of alleged "additional criminal conduct" including "a series of bank frauds and bank fraud conspiracies."
On Thursday, those new discoveries re-materialized in the form of a 32-count superseding indictment against Gates and Manafort.
The same day, Judge Amy Berman granted a motion allowing Gates' legal counsel comprising of lawyers Shanlon Wu, Annemarie McAvoy and Walter Mack, to withdraw as his defense team. The lawyers initially made a motion to withdraw on Feb. 1, fueling speculation at the time that Gates was perhaps unable to pay his legal fees, or that he may have lied or planned to lie in court.
Multiple outlets also reported that Gates had recently participated in a so-called Queen for a Day interview, in which he would provide any and all information asked by investigators without any of his admissions being used against him.
Shortly before his counsel withdrew, Gates retained Thomas Green, a defense lawyer who had represented clients in the Watergate, Iran Contra and Whitewater political scandals.
But Gates may have already terminated Green, according to a Thursday report from the Daily Beast, which said Green has been replaced by Barry Pollack of the law firm Miller Chevalier. A Buzzfeed reporter later tweeted that Green told him he was still representing Gates.
--CNBC's Bria Horsley contributed to this report.