Secretary of State John Kerry called the verdict, in which two octogenarian leaders of the 1975-1979 regime were handed life sentences, "a historic, if long delayed, step along the path for Cambodia."
"Cambodians have received a small measure of justice and a reminder that justice may not be swift, but justice is resolute," Kerry said, in a statement.
Kerry said that the United States would maintain support to the UN-backed special tribunal, which is financed by foreign governments -- with Japan by far the largest donor.
Kerry, who served in the Vietnam War, said he was personally committed to supporting accountability over the "Killing Fields" era in which the Khmer Rouge's peasant army waged a radical campaign to restructure society that killed up to two million people.
While contributing aid, the United States has been critical on human rights grounds of Prime Minister Hun Sen, who has ruled Cambodia for nearly three decades.