The sentencing date for Emmanuel, who faces life in prison, was set for January 9.
Emmanuel, 31, was the head of Liberia's anti-terrorism taskforce during his father's rule.
Emmanuel's job from 1999 to 2002 was to use his paramilitary force to silence opposition to his father and to train soldiers for conflict in neighbouring African countries, the court heard earlier.
Taylor is currently on trial at a special UN-backed court in the Netherlands for suspected war crimes committed during the civil war in Liberia's neighbour, Sierra Leone.
Alexander Acosta, south Florida's attorney general, said some of the witnesses testifying in the US trial still carried scars left by their torture.
"The acts of which [Emmanuel] was convicted were horrific," he said. "It's the first [case] of its kind - but that doesn't mean it's the last of its kind."
The trial comes under a 1994 US law that allows prosecution of US nationals charged with torture outside the United States.
The rights group Human Rights Watch (HRW) lauded the verdict, calling it a "milestone in ensuring justice for atrocities".
Elise Keppler, a senior lawyer for HRW's justice programme, said: "Never before has torture committed abroad been prosecuted in the United States.
"We now look to the department of justice to bring more cases like this one."
Emmanuel, born in the US state of Massachusetts, was taken into custody in March two years ago while trying to enter the United States from Trinidad.
He initially pleaded guilty to committing passport fraud.
In December 2006, on the day before his sentencing, he was charged with carrying out torture.
Ellen Johnson Sirleaf, Liberia's current president, said in a visit to the US last week that her country was beginning to recover from the ravages of war.
"We've been able to put our economic and financial house in order, tackle our debt, begin to rebuild our infrastructure, put our children back into school," she said in Washington, DC.
"We feel that Liberia can become a post-conflict success story."