In a 6-3 opinion, written by Justice Anthony Kennedy, the court said that its 2012 ruling that banned mandatory sentences of life without parole for juvenile offenders applies retroactively, because it represents a new substantive rule.
Lawyers for both sides believe the ruling could affect at least 1,000 similarly situated inmates across the country.
The case was brought by Henry Montgomery, 69, who has been serving a life-without-parole sentence that he received in 1963 as a juvenile for the murder of Sheriff Deputy Charles Hurt in East Baton Rouge, Louisiana. He had asked the justices to take a case decided four years ago called Miller v. Alabama and apply it to his case, even though he committed his crime well before that ruling. In Miller, the court held that mandatory sentences of life without parole for juvenile homicide offenders violate the Constitution.
The court's ruling Monday does not overturn Montgomery's conviction, but it allows him either a new sentencing hearing or a new parole hearing.
"Children are constitutionally different from adults for purposes of sentencing" because they have diminished culpability and greater prospects for reform, Kennedy wrote in his majority opinion.
Kennedy said that Montgomery has spent "each day of the past 46 years" knowing he was condemned to die in prison. "Perhaps it can be established that, due to exceptional circumstances, this fate was a just and proportionate punishment for the crime he committed as a 17 year old boy," Kennedy said. But he added that prisoners like Montgomery "must be given the opportunity to show their crime did not reflect irreparable corruption; and, if it did not, their hope for some years of life outside prison walls must be restored.
"Allowing those offenders to be considered for parole ensures that juveniles whose crimes reflected only transient immaturity — and who have since matured — will not be forced to serve a disproportionate sentence in violation of the Eighth Amendment," Kennedy wrote.
Scalia compares majority opinion to 'The Godfather'
Justice Antonin Scalia wrote a dissent joined by Justice Clarence Thomas and Justice Samuel Alito. Scalia said the court did not have jurisdiction to hear the case and, "the decision it arrives at is wrong."
"Once a conviction has become final, whether new rules or old ones will be applied to revisit the conviction is a matter entirely within the state's control," Scalia wrote. "The Constitution has nothing to say about that choice."
Scalia said the majority, "in Godfather fashion," offers states legislatures an "offer they can't refuse" by telling them they can either revisit some cases that existed decades ago, or allow the juvenile offenders to be considered for parole.
Scalia called it a "devious way of eliminating life without parole for juvenile offenders."
Unusual for court to apply rulings retroactively
Montgomery's lawyers had argued that he was incarcerated "based upon a constitutionally disproportionate sentence that could not be imposed today."
"I think it is a great day for Mr. Montgomery," said Marsha Levick of the Juvenile Law Center, who was one of his lawyers. "He's been in prison for more than five decades, he has by all accounts demonstrated that he has matured and been rehabilitated and this decision gives him hope that he may yet again live the remainder of his life outside of prison walls."
A lawyer for the State of Louisiana argued that Miller did not apply to Montgomery in part because the Miller opinion was an "incremental step in sentencing juveniles."
In general, the Court's decisions on criminal law don't normally apply retroactively. But court precedent allows exceptions if the new rule is considered substantial.
S. Kyle Duncan, a lawyer representing Louisiana, argued that in Miller, the Court had left sentences of life without parole on the table, and said that if the Court were to apply the case retroactively, it would require the re-litigation of facts that over time have been lost.
Lawyers for the Obama administration argued in support of Montgomery.
Hurt, the sheriff's deputy, left behind a wife and three young children. One of his daughters, Becky Wilson, filed a brief in support of Louisiana. Her lawyers wrote that if the court were to rule in favor of Montgomery, surviving family members of the victims will be forced to "relieve the events that traumatized them in the first instance."