Even though the Manafort case was tangential to the central thrust of Mr Mueller's inquiry - relating to his work as a lobbyist, and not possible ties between the Russian government and the Trump campaign - this still marks a significant development in the special counsel's investigation.
There was some concern about the risk to Mr Mueller's investigation faced going into the Manafort trials.
According to documents released with his guilty plea, Papadopoulos offered to help set up a meeting with then-candidate Trump and Russian President Vladimir Putin.
The once-powerful Republican operative was found guilty last month of tax and bank fraud charges that could land him in prison for up to 80 years. Prosecutors dropped five other charges that included lobbying violations and money laundering.
Manafort was instrumental to President Donald Trump's securing the GOP nomination in 2016 and served on his campaign until August of that year.
"I had hoped and expected my business colleague would have had the strength to continue the battle to prove our innocence", Manafort said in February.
The others are Michael Cohen, former campaign adviser Michael Flynn, deputy chairman Rick Gates, attorney Alex van der Zwaan and businessman Richard Pinedo.
Prosecutor Andrew Weissmann told the judge that Manafort's plea deal is a "cooperation agreement". He is now accused of conspiracy against the United States and conspiracy to obstruct justice. And last month Trump praised Manafort for being "brave" and refusing to "break" to take a plea deal.
Manafort also agreed not to appeal any trial or pre-trial issues in his Eastern District of Virginia conviction, and "abandons any interest he has in all forfeitable property and consents to any disposition of the property by the government without further notice" - to the tune of $46 million, something any presidential pardon could not retrieve.
Manafort was convicted in Virginia on charges that pre-dated his stint on the Trump campaign and involved his work with pro-Russian politicians in Ukraine.
He pleaded guilty to conspiracy to obstruct justice against the United States and for money laundering, tax fraud, bank reporting and FARA foreign agent disclosure. It has not been established that Trump himself knew about this meeting at the time, but Manafort might be able to testify to that as well.
Twelve Russian intelligence officers were indicted by a federal grand jury on July 13, accused of hacking Democratic Party computer networks in 2016, in the most detailed USA accusation yet that Moscow meddled in the presidential election to help Trump.
"He knows nothing harmful to the president and the plea is the best evidence of that", Rudy Giuliani, the former New York City mayor who is representing Trump in the Russian Federation probe, told Reuters. He had been jailed since June as a result of the witness tampering charges.
- Super Smash Bros. Ultimate adds Isabelle
- Read Ariana Grande’s Tribute to Mac Miller
- Justin Bieber & Hailey Baldwin Stop By Marriage Bureau in NYC!
- X-Men Actress Fan Bingbing is Missing
- European Parliament backs copyright changes
- Tesla narrows paint options to simplify production
- West Nile fever virus claims four more lives in Serbia
- A tricked-out iPhone XS Max costs more than a MacBook
- News Corp Stands By Serena Williams Cartoon
- Hurricane Florence unleashes wrath on USA , with worst yet to come