Thirty-nine executions were carried out in nine states, down from 43 last year and only the second time in 19 years there have been fewer than 40 executions, the Death Penalty Information Center said.
Eighty death sentences were handed down, two more than last year but still far below the peak of 315 seen in 1994 and 1996.
"The key reason for the decline in the death penalty has been the revelation of so many mistakes," said the center's executive director Richard Dieter.
"Over 140 people have been exonerated and freed from death row, including another inmate in 2013," he said in an email to AFP.
He added: "Juries are less likely to impose the death penalty and prosecutors are less likely to seek it."
"Life without parole, which is now quite prevalent, has become the alternative to the death penalty and will likely replace it completely in the future."
The United States ranks fifth in the top 10 list of nations with the most executions between 2007 and 2012, according to Amnesty International.
China tops the table with "thousands" of executions, followed by Iran and Saudi Arabia, the human rights group has said.
Of the 3,108 inmates on death row around the United States, the biggest number are in California (731) followed by Florida (412) and Texas (298).
Citing an annual Gallup poll, the center said popular support for the death penalty among Americans is, at 60 percent, at its lowest level in four decades.
Forty percent don't believe the death penalty is administered fairly — and after the Boston Marathon bombing in April, only one in three respondents to a Boston Globe poll thought the sole living suspect, Dzhokhar Tsarnaev, should be executed if convicted.
Thirty-two of the 50 US states have the death penalty, after Maryland earlier this year became the sixth in six years to end capital punishment. But the majority of executions in 2013 took place in two states — Texas (16) and Florida (seven).
"If crime were to suddenly spike upwards, the public would demand more punishment. But the problems of the death penalty will remain," Dieter said.
"Once it is abolished, it rarely comes back. The demise of the death penalty in the United States is a state-by-state decision, and that process is likely to continue for the near future."
In its report, the center mentioned another reason for fewer executions — frustration in finding a consistent way of conducting an execution through lethal injection.
Many drugs used in such injections are made in Europe, but their export to the United States for use in execution is banned.
Some states such as Georgia, Missouri, Ohio and Texas have turned to tailor-made drugs from compounding pharmacies, but such facilities are not subject to federal drug regulators, opening the door to legal challenges from death-penalty opponents.
Arkansas's attorney general Dustin McDaniel has told sheriffs that "essentially pointless litigation" might ultimately lead to either the demise of the death penalty or changes to the method of execution.
The UN General Assembly called in 2007 and 2008 for a worldwide moratorium on the death penalty — an EU initiative that the United States voted against alongside the likes of China, North Korea and Iran.