Riot police had lined up outside the court on the outskirts of the capital, Phnom Penh, on Monday as Duch arrived in a bulletproof car.
The 67-year-old remained impassive as the verdict was read out, but relatives of victims burst into tears, disappointed with the length of the prison term.
'Insult to victims'
Theary Seng, a human rights lawyer whose both parents were killed by the Khmer Rouge, told Al Jazeera that the sentence was "an insult to victims".
"It's not acceptable that a man who is responsible for killing at least 12,000 people, possibly 16,000, is only serving 19 years. What is that, eleven hours per murder?"
The court said it opted against life in prison for several reasons, including Duch's expressions of remorse, co-operation with the court, his "potential for rehabilitation" and the coercive environment of life under the Khmer Rouge.
"The chamber has decided there are significant mitigating factors that mandate a finite term imprisonment rather than life imprisonment," the tribunal's president said in a statement.
Al Jazeera's Harry Fawcett, reporting from the court, described the sentence as "a very big moment in Cambodian history".
"It's the first time since the fall of the regime 31 years ago that anyone responsible in any way for even a small number of the grave atrocities committed during that time has been held accountable," he said.
Anne Heindel, a legal adviser for the Documentation Centre of Cambodia, said people around Cambodia had followed the court proceedings with great interest.
"We have set up screens in seven provinces so that people can watch the live footage," she told Al Jazeera.
"There is a great deal of interest because even though these events are only limited to one detention centre it really reflects on all the detention centres throughout the country and events that happened to other people, even if in other contexts."
Begged for forgiveness
The verdict was the first by the court, established in 2006 after nearly a decade of negotiations between the government and the UN.
Duch is the only senior Khmer Rouge figure to have acknowledged responsibility to the tribunal.
Al Jazeera's Harry Fawcett visits the former Tuol Sleng prison which was run by Duch
During nine months of hearings last year, he repeatedly begged forgiveness for overseeing the murders of around 14,000 people at the Tuol Sleng torture centre over three decades ago.
But the former math teacher then asked to be released on the final day of hearings on grounds that he was not a key leader in the government and was only following orders.
The Khmer Rouge, led by "Brother Number One" Pol Pot, emptied Cambodia's cities during its 1975-1979 rule, exiling millions to vast collective farms in a bid to take society back to "Year Zero" and forge a Marxist utopia.
An estimated two million people were killed in the notorious "Killing Fields" or died from starvation and overwork before a Vietnamese-backed force toppled the government.
Pol Pot died in 1998 but four other Khmer Rouge leaders, all said to be more senior than Duch, are in custody awaiting trial.
"Brother Number Two" Nuon Chea, former head of state Khieu Samphan, the former foreign minister Ieng Sary and his wife Ieng Thirith, who was the minister of social affairs, are expected to go on trial next year.
Duch has been detained since 1999, when an investigative journalist found him working as a Christian aid worker in the jungle.