The board announced its decision Monday after hearing arguments in the case Friday. Hill was convicted in 1991 and sentenced to death for killing a fellow inmate while serving a life sentence for the slaying of his girlfriend.
His lawyer Brian Kammer has argued that Hill is mentally disabled and therefore shouldn't be executed. Kammer said he's "horrified and outraged" by the board's decision.
"This shameful decision violates Georgia's and our nation's moral values and renders meaningless state and federal constitutional protections against wrongful execution of persons with mental retardation," he said.
Kammer had asked the board to commute Hill's sentence to life in prison without parole or to grant him a 90-day stay of execution to give the U.S. Supreme Court time to consider the case. A petition to have Hill's case heard by the U.S. Supreme Court was denied last month, but Kammer has filed a new request with the high court.
Hill and Joseph Handspike were both serving sentences for murder at the Lee Correctional Institution in 1990 when Hill beat Handspike to death. Hill was serving a life sentence at the time for the 1986 slaying of his 18-year-old girlfriend, who was shot 11 times.
Hill's defense says he is mentally disabled and therefore shouldn't be executed because state and federal law prohibit states from executing people who are mentally disabled. The state has said the defense has failed to meet its burden of proving beyond a reasonable doubt that Hill is mentally disabled.
Georgia's standard of requiring death row inmates to prove mental disability beyond a reasonable doubt is the toughest in the country. Most states that impose the death penalty have a lower threshold for defendants to prove they are mentally disabled, while some states don't set standards at all.
Kammer has said the high standard for proving mental disability is problematic because psychiatric diagnoses are subject to a degree of uncertainty that is virtually impossible to overcome. But Georgia's strict standard has repeatedly been upheld by state and federal courts.