An American lobbyist on Friday admitted brokering access to President Trump’s inauguration for a pro-Russian Ukrainian oligarch in a scheme that highlighted the rush by foreign interests to influence the new administration.

As part of a plea agreement under which he pledged to cooperate with federal prosecutors, the lobbyist, Sam Patten, pleaded guilty to failing to register as a foreign agent for a Russia-aligned Ukrainian political party, and to helping the Ukrainian oligarch who had funded that party illegally purchase four tickets to Mr. Trump’s inauguration.

Although the charges were not brought by the special counsel investigating Russian interference in the 2016 presidential election, Robert S. Mueller III, they stem from his team’s work, and overlap substantially with its continuing investigation, suggesting that Mr. Patten could be a useful witness.

The case sketched out by prosecutors encompassed Mr. Patten, a respected Republican operative and consultant whose family was once part of Washington’s social elite; money transfers from a Cypriot bank; and a Russian national who had also worked for Paul Manafort, Mr. Trump’s former campaign manager, and been accused of maintaining ties to Russian intelligence.

The charges against Mr. Patten also represented the first public acknowledgment that prosecutors are looking into efforts by foreign interests to funnel money into Mr. Trump’s political operation, including his inaugural committee.

Prosecutors did not present any evidence that the inaugural committee was aware of the scheme. Questions to representatives of the inaugural committee on Friday about the illegal purchase of tickets went unanswered.

Mr. Trump’s inauguration came at a moment when some international figures were paying large sums to Mr. Trump’s associates in an effort to curry favor with an administration that came into office with few ties to established players on the world stage, and with an uncertain foreign policy agenda. That was particularly true in matters related to Russia and its neighbors.

During his campaign, Mr. Trump flouted traditional foreign policy by praising President Vladimir V. Putin of Russia and suggesting that if elected he might be willing to ease sanctions levied against Russia over its aggression against Ukraine.

That alarmed allies of Petro O. Poroshenko, the Ukrainian president, who is generally regarded as more pro-Western, and it emboldened his pro-Russian opponents, who had held power from 2010 to 2014 partly through the efforts of Mr. Manafort and his former right-hand man, Rick Gates.

Forced into the opposition, the oligarchs formed a political party called Opposition Bloc, which worked with Mr. Manafort and Mr. Gates in 2014, before largely shifting its political consulting and lobbying business to a company started by Mr. Patten and Konstantin V. Kilimnik. He is a Russian national who had worked closely with Mr. Manafort in Ukraine and is suspected by the United States of having ties to Russian intelligence.

The company was paid more than $1 million through an offshore Cypriot bank account to advise the party and its members, including a “prominent Ukraine oligarch” between 2014 and this year, according to a court document filed on Friday.

Prosecutors contend that Mr. Patten should have disclosed the work, which included lobbying and public relations assistance intended to “influence United States policy,” with the Justice Department pursuant to the Foreign Agents Registration Act.

Mr. Patten also attended an unspecified inaugural event with the oligarch, according to prosecutors. They contend that Mr. Patten arranged for the purchase of four tickets for $50,000 by funneling the oligarch’s money through an American “straw” purchaser in violation of rules barring the inaugural committee from accepting money from foreign nationals.

Mr. Patten intentionally withheld documents from the Senate Intelligence Committee that could have revealed that foreign money was used to purchase the tickets, according to court filings.

Mr. Patten’s lawyer declined to comment after a Friday court hearing in which Mr. Patten entered his guilty plea. He faces up to five years in prison and a fine of up to $250,000. He was released after the hearing and awaits sentencing.

Prosecutors did not identify the American who purchased the tickets, the oligarch or Mr. Patten’s Russian business partner by name.

But people familiar with the work said the Russian is Mr. Kilimnik, who formed a company called Begemot Ventures International with Mr. Patten in February 2015, according to corporate records filed in Washington. Reports Nytimes