Patten pleaded to the count before Judge Amy Berman Jackson in U.S. District Court for the District of Columbia.
Patten was a business associate of Konstantin Kilimnik, who has ties to Russian intelligence.
Manafort was recently convicted on eight counts of tax fraud, bank fraud, and failure to report foreign bank accounts. Mr. Kilimnik was indicted in June on charges of obstruction of justice related to witness tampering in the foreign-lobbying case against Mr. Manafort.
Court documents say Patten drafted op-eds on behalf of an unnamed foreigner and successfully placed an op-ed in an unnamed American media outlet in February 2017 that focused on "Ukraine's ability to work effectively with the new United States administration".
Washington lobbyist W. Samuel Patten pleaded guilty Friday to acting as an unregistered foreign agent in the United States for Ukrainians from 2014 until 2017.
The charges against Patten come after Ryan Dickey and Brian Richardson, a pair of "relatively junior" special counsel prosecutors, have left Mueller's team.
U.S. Special Counsel Robert Mueller's office referred the case to the U.S. Attorney in the District of Columbia, according William Miller, a spokesman for U.S. Attorney Jessie Liu.
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The case against Patten was referred to the USA attorney's office in DC by Special Counsel Robert Mueller, whose team has been investigating potential coordination between Moscow and the Trump campaign.
No sentencing date has been set.
Manafort made most of his money working for former Ukrainian President Viktor Yanukovych before he fled to Russian Federation in 2014, but like Patten also sought to drum up business with the Opposition Bloc in the aftermath of Yanukovych's exit.
Patten then attended an inauguration event with the Ukrainian client, the document said.
Patten also carried out work for Cambridge Analytica, the now-defunct consultancy that is under scrutiny for its work on Trump's 2016 election campaign.
In court documents, prosecutors allege that Patten also worked with the two foreigners to help "Foreigner B" make an illegal contribution to Trump's Inauguration. He faces up to five years in prison and a $250,000 fine.
Patten performed various services for Cambridge Analytica, the company that became embroiled in controversy earlier this year due to its sweeping collection of Facebook data, as well as its parent company SCL Group.
Patten's long friendship with Kilimnik-which stems from their time working together at the International Republican Institute in Moscow between 2001 and 2003-would likely be enough to draw scrutiny from Mueller, who appears to have homed in on Kilimnik as a potentially significant link between the Trump campaign and Russian Federation. Prosecutions of the offense are rare, but in recent years the Justice Department's national security division has taken a tougher stance on enforcement of the law. But this was not a case brought by special counsel Robert Mueller.