Kilimnik is also a close business associate of President Donald Trump's former campaign chairman Paul Manafort, who also did political work in Ukraine and has been found guilty on eight counts of financial crimes.
Patten told Judge Amy Berman Jackson that he suffers from mental illness and is under the care of a psychiatrist and is taking medication.
In 2014, Patten and a Russian national formed a company in the US that offered lobbying and political consulting services.
Although the charges were not brought by Robert Mueller, the special counsel investigating Russian interference in the 2016 presidential election, they stem from his team's work, and overlap substantially with its continuing investigation, suggesting Patten could be a useful witness.
From 2014, Patten provided a "prominent" Ukrainian oligarch, who isn't named in court papers, and his Opposition Bloc political party with lobbying and consulting services, according to the criminal information.
Patten, 47, was charged with violating the Foreign Agents Registration Act (Fara) by failing to register with the United States government as an agent for a foreign country.
W. Samuel Patten appeared in U.S. District Court for the District of Columbia where he entered a guilty plea to one count of failing to register as a foreign agent. They were lead Manafort prosecutor Andrew Weissmann and Federal Bureau of Investigation agents Omer Meisel and Brock Domin.
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In court, Patten conveyed the air of a solemn man aware of what he had done. His sentencing date has not yet been scheduled.
Patten said previous year that Kilimnik had been essential to Manafort's Ukraine operation, which had been perceived by allies and opponents alike as the savviest of the American consulting operations working in the country. He faces up to five years in prison and a fine of up to $250,000.
He then misled the Senate Intelligence Committee about the tickets while testifying in January by withholding documents that would've revealed the money trail back to a foreign account.
Patten's ties to Kilimnik are more concrete.
He headed the International Republican Institute's Moscow office from 2001 to 2004, and provided technical assistance to a range of political parties and non-governmental organizations, according to the website.
Working alongside an unnamed Russian national, Patten set up meetings with members of Congress and their staff, as well as the executive branch, in 2015, according to the document.
Prosecutions of the offense have been rare, but in recent years, the Justice Department's national security division has taken a tougher stance on enforcement of the law.
Even though Trump and his legal team have indicated that they would wait until after special counsel Robert Mueller's investigation wraps up, the pardoning of Paul Manafort reportedly is, and will continue to be on the table. Patten also drafted an op-ed for Foreigner B that sought to address concerns about Ukraine's ability to work with the Trump administration.