Former police officer Derek Chauvin, 45, was found guilty of murder over the death of African-American George Floyd on a Minneapolis street last year.
Chuvin was filmed kneeling on Mr Floyd’s neck for more than nine minutes during his arrest last May.
The widely watched footage sparked worldwide protests against racism and excessive use of force by police.
Chauvin was found guilty by a US jury on three charges: second-degree murder, third-degree murder and manslaughter.
He will remain in custody until he is sentenced and could spend decades in jail.
The 12-member jury took less than a day to reach their verdict, which followed a highly-charged, three-week trial that left Minneapolis on edge.
After both sides presented closing arguments on Monday, the jury was isolated in a hotel with no outside contact so they could deliberate on a verdict, a process known as sequestration.
Jurors had to agree unanimously and were told they could not return home until they had made their decision.
The verdict prompted celebratory scenes outside the court, where several hundred people cheered as it was announced.
The Floyd family’s lawyer, Ben Crump, said it marked a “turning point in history” for the US.
“Painfully earned justice has finally arrived,” he tweeted. [It] sends a clear message on the need for accountability of law enforcement.”
President Joe Biden and Vice-President Kamala Harris called the Floyd family right after the verdict.
Mr Biden was heard saying that “at least now there is some justice”.
“We’re gonna get a lot more done. This is going to be a first shot at dealing with genuine systemic racism,” the president said.
And in a nationally televised remarks shortly afterwards, Mr Biden said: “Systemic racism is a stain on the whole nation’s soul.”
Meanwhile, Ms Harris urged lawmakers to pass the George Floyd bill aimed at reforming policing in the US. “This bill is part of George Floyd’s legacy. This work is long overdue,” she said.
Chauvin is expected to appeal against the verdict.