A majority of justices refused an emergency request by the governor to halt a lower court's directive for the early release of the prisoners to ease severe overcrowding at California's 33 adult prisons.
The decision was met with concern by law enforcement officials in the state.
Covina Police Chief Kim Raney, president of the California Police Chiefs Association, said the justices ignored efforts already underway to reduce prison populations and "chose instead to allow for the release of more felons into already overburdened communities."
Brown's office referred a request for comment to the California Department of Corrections and Rehabilitation, where Secretary Jeff Beard vowed that the state would press on with a still-pending appeal in hope of preventing the releases.
A panel of three federal judges had previously ordered the state to cut its prison population by nearly 8 percent to roughly 110,000 inmates by Dec. 31 to avoid conditions amounting to cruel and unusual punishment. That panel, responding to decades of lawsuits filed by inmates, repeatedly ordered early releases after finding inmates were needlessly dying and suffering because of inadequate medical and mental health care caused by overcrowding.
Court-appointed experts found that the prison system had a suicide rate that worsened last year to 24 per 100,000 inmates, far exceeding the national average of 16 suicides per 100,000 inmates in state prisons.
Brown had appealed the latest decision of the panel and, separately, asked the U.S. Supreme Court to cancel the early release order while considering his arguments that the state is making significant progress in improving conditions. The high court refused Friday to stop the release but did not rule on the appeal itself. Corrections Secretary Beard said the state would press on with that, so the "merits of the case can be considered without delay."
However, inmate lawyer Don Specter, head of the Berkeley-based Prison Law Office, said the ruling Friday did not bode well for the overall appeal. He said the decision underscores what inmates have been arguing for years.
"The conditions are still overcrowded," he said. "The medical and health care remain abysmal."
Lawyers representing Brown had argued to the high court that releasing 10,000 more inmates would mean letting violent criminals out on the streets and overwhelm the abilities of law enforcement and social services to monitor them.
"No data suggests that a sudden release of inmates with these characteristics can be done safely," the state said in its filing. "No state has ever done it."
The panel of federal judges has consistently rejected that argument. The judges, prisoners' lawyers and others say other states have marginally reduced inmate sentences without sparking an increase in crime.
The governor said the state has already transferred thousands of low-level and nonviolent offenders to county jails, but that local officials in turn have been forced into releasing some inmates early to ease their own overcrowding issues.
The Supreme Court's ruling rejected Brown's plea over the objections of Justices Samuel Alito, Antonin Scalia and Clarence Thomas, who all said they would have granted the state's request.
Scalia, in a dissent joined by Thomas, wrote that the previous order by the three-judge panel was a "terrible injunction" that threatens public safety. Scalia said the state's evidence shows it has made meaningful progress and that such reductions in the inmate population are no longer necessary.
The legal battle goes back years. In 2011, the U.S. Supreme Court ruled 5-4 that California had to cut its inmate population to deal with unconstitutional prison conditions caused by overcrowding. It said that further delay in reducing prison overcrowding would further the substandard delivery of medical and mental health care and, by extension, lead to more inmate deaths and injuries.
In recent years, the special panel of federal judges accused Brown of attempting to delay and circumvent their orders. They previously threatened to cite the governor for contempt if he did not comply.
The judges waived all state laws in June as they ordered Brown to expand good-time credits leading to early release. They also directed the governor to take other steps, including sending more inmates to firefighting camps, paroling elderly felons, leasing cells at county jails and slowing the return of thousands of inmates now housed in private prisons in other states.
If those steps fail, the judges ordered the state to release by year's end enough inmates from a list of lower-risk offenders until it reaches the maximum allowed population.
In its latest filing with the Supreme Court, the state argued that no governor has the unilateral authority to take the steps ordered by the three-judge panel. That would require approval by the Legislature or judicial pre-emption of California's core police powers, the administration argued.
Brown has said the state is spending $2 billion on new or expanded facilities for inmate medical and mental health treatment. That includes seven new centers for mental health treatment and the opening last June of an $839 million prison hospital in Stockton that will treat 1,722 inmates requiring long-term care. The state also has boosted hiring and salaries for all types of medical and mental health professionals.
The state has already reduced the population by 46,000 inmates since 2006.
More than half of the decrease that has occurred so far is due to a two-year-old state law - known as realignment - that is sentencing offenders convicted of crimes considered nonviolent, non-serious and non-sexual to county jails instead of state prisons.