Papadopoulos, who served as a foreign policy adviser to President Trump's campaign, has been a central figure in the Russian Federation investigation dating back before Mueller's May 2017 appointment.
Papadopoulos served as a foreign policy adviser to President Trump's 2016 campaign, and was the first to plead guilty in the Mueller probe.
The sentence was imposed in federal court in Washington.
After the Clinton emails were leaked, the Australians told U.S. intelligence counterparts what Downer had heard, spurring the Federal Bureau of Investigation to begin investigating possible collusion between the Trump campaign and Moscow.
President Donald Trump and his allies have repeatedly downplayed Papadopoulos' role on the campaign.
In Friday's sentencing, Papadopoulos was also handed 12 months of supervised release, 200 hours of community service, and a fine of $9,500 (£7,350).
The operatives Papadopoulos met offered him "dirt" on Hillary Clinton and "off-the-record" meetings in overtures he discussed with leaders of Donald Trump's campaign. The conversations actually occurred after he was brought on as a foreign policy adviser. In late April, he told them that Mifsud said the Russians had information that could harm Clinton, in the form of thousands of emails.
The president told reporters on Air Force One on Friday that "I don't know Papadopoulos". In a court filing August 31, Papadopoulos's lawyers painted him as a political newcomer "eager to show his value to the campaign", and quickly got in over his head.
His lawyers say Papadopoulous acted out of a "misguided sense of loyalty to his master" and to preserve his career options in the new administration.
The FBI met with Mifsud while he was visiting the United States in early 2017, but Papadopoulos' lies "undermined investigators' ability to challenge the professor or potentially detain or arrest him". "I don't know him", Trump said.
His lawyers said Papadopoulos had suggested brokering a meeting between Trump and Russian President Vladimir Putin in 2016 and that Trump "nodded with approval", according to a court filing last month.
In addition, they said Mr. Papadopoulos did not provide "substantial assistance" and only came clean after he was confronted with his own e-mails, texts and other evidence.
Even after his arrest and plea agreement previous year, prosecutors say Papadopoulos continued to be hard with investigators, only providing information after being confronted with documents such as emails and text messages.
His attorneys say he cooperated fully.