Ex-Trump campaign chief Paul Manafort has been found guilty on eight charges of tax fraud, bank fraud and failing to disclose foreign banks accounts.
But the jury at his trial in Virginia failed to reach a verdict on 10 other charges and the judge declared a mistrial on those.
It was the first criminal trial arising from the justice department probe into alleged Russian election meddling.
But none of the charges relate to collusion with Russia in the election.
Responding to the charges, President Donald Trump said that Manafort's conviction was part of a "witch hunt" following the 2016 presidential election.
"It's a very sad thing that happened," Mr Trump told reporters as he arrived at a rally in West Virginia on Tuesday evening, adding that it had "nothing to do with Russian collusion".
The verdict came in the same hour that former Trump lawyer Michael Cohen admitted violating campaign finance laws in relation to hush money paid to Mr Trump's alleged mistresses.
Manafort, 69, was a key figure in Mr Trump's inner circle, but the president has since sought to distance himself from his former adviser.
The jury came back on the fourth day of deliberations. The judge has given prosecutors until 29 August to decide whether to ask for a fresh trial on the charges the jury was deadlocked on.
A lawyer for Manafort, Kevin Downing, said his client was "disappointed" with the verdict and was "evaluating all of his options at this point".
A date for sentencing has yet to be set.
Will it leave a scar, or just a stain?
Analysis by BBC's North America Editor Jon Sopel
It was as though the verdicts had been synchronised by a Hollywood producer.
Within two minutes of each other a federal courtroom in New York heard Michael Cohen plead guilty to eight charges concerning fraud and violating campaign finance laws, and in Alexandria in Virginia a federal court found Donald Trump's onetime campaign manager, Paul Manafort, guilty on eight counts of tax and banking fraud. And this all happened as Mr Trump flew to a rally in West Virginia.
When he landed the president said he felt "very sad" for Mr Manafort. About Michael Cohen he said nothing. And that is instructive.
Mr Manafort's crimes predate his association with Mr Trump, and is easy to deal with. But the guilty plea of Mr Cohen is altogether more toxic. In saying he broke campaign law in co-ordination with and at the direction of a candidate for federal office, he is stating in no uncertain terms that Mr Trump was a co-conspirator. That leaves a stain, but does it leave a scar?
This president has shown an unerring ability to shake off scandals that would have sunk many others. Will his audience in West Virginia be appalled at Tuesday's courtroom drama, or enthused by a buoyant economy and a president who they believe is fighting for them? Probably the latter.
But independents and traditional Republicans might be less impressed - and that could cause Mr Trump a whole heap of problems come the US midterm elections in November.
What happened during the trial?
Prosecutors presented evidence of Manafort's luxurious lifestyle - including photos of a $15,000 (£11,000) ostrich coat.
Judge TS Ellis scolded the prosecution for spending too much time on Mr Manafort's wealth, saying it "isn't a crime to have a lot of money".
But prosecutors had emphasised Manafort's extravagant spending was only possible because of his bank and tax fraud.
He was accused of using 31 foreign bank accounts in three different countries to evade taxes on millions of dollars.
The prosecution's star witness was Manafort's former associate and fellow Trump campaign staffer Rick Gates.
Gates has admitted embezzling hundreds of thousands of dollars from Mr Manafort while allegedly helping his boss commit tax crimes.
The defence sought to undercut Gates' credibility by citing his "secret life", and noting that he had told so many lies he could not keep track of them all.
The judge also said he had received death threats related to the Manafort case and was under US Marshal Service protection.
He added that he would not release jurors' names due to safety concerns.
How did we get here?
Mr Manafort worked for Mr Trump's presidential campaign for five months in 2016, and he was in charge when Mr Trump clinched the Republican party nomination.
Last October, Mr Manafort was charged by special counsel Robert Mueller, who was appointed by the Department of Justice to investigate alleged Russian meddling in the 2016 election.
The Virginia trial was not connected to the US president, but instead dealt with Mr Manafort's political consulting with pro-Russian politicians in Ukraine, largely predating his role with the Trump campaign.
The Ukrainian government found ledgers pledging more than $12m (£9.2m) in cash to Manafort for his advisory work with former President Viktor Yanukovych while he was in office, the New York Times reported.
It was also alleged that Mr Manafort secretly worked for a Russian billionaire to aid Russian President Vladimir Putin's political goals in other parts of the former USSR. Mr Manafort denied that allegation.