Before George Papadopoulos pleaded guilty to lying to federal agents about contacts with Russians in 2016, he played a starring role in Europe as a foreign-policy surrogate for then-presidential candidate Donald Trump.

He drew headlines in Great Britain for criticizing U.K. Prime Minister David Cameron in May 2016 over comments that Mr. Trump was “divisive, stupid and wrong.” “An apology or some sort of retraction should happen,” Mr. Papadopoulos told the Times of London.

That spring, Mr. Papadopoulos traveled to Greece, presenting himself to officials including the Greek president and foreign minister as a senior campaign official, Greek officials said. Later that year, he dined in an Athens fish tavern with a far-right political leader, Panos Kammenos.

“He was also saying he would be able to grant access to the Trump administration after his election,” one Greek official said.

Since Mr. Papadopoulos emerged this week as a central figure in Special Counsel Robert Mueller’s Russian election-meddling probe, Trump campaign and administration officials have said he was an unimportant adviser whose ideas were consistently rebuffed. Regarding Mr. Papadopoulos, Mr. Trump said in a tweet Tuesday: “Few people knew the young, low level volunteer named George, who has already proven to be a liar.”