The unanimous ruling from the Supreme Court of Georgia now allows the state to put Warren Lee Hill to death and resume executing other prisoners on death row.
It was not immediately clear how soon Hill could be executed. Typically, the Georgia Department of Corrections receives an order from a judge and a timeframe for the execution is then known, agency spokeswoman Gwendolyn Hogan said.
Monday's unanimous ruling found that the decision to replace a three-drug cocktail with one drug for executions is not subject to the Georgia Administrative Procedure Act, so changing the execution method does not require public hearings.
Hill had accused prison officials of violating state procedures by failing to hold a public hearing before changing the execution procedure.
The ruling lifts the stay of execution of the Supreme Court of Georgia had granted to Hill in July, hours before he was to die by lethal injection for the 1990 killing of a prison inmate.
Hill was set to be executed July 18. The day before, the Department of Corrections announced the change in its execution protocol and delayed Hill's execution until July 23.
Around that time, there was a nationwide shortage of one of the drugs, pancuronium bromide.
The unavailability of pancuronium bromide was a factor in the decision to change Georgia's execution protocol, but it was not an "overriding concern," the Department of Corrections later said in a statement.
Hill's lawyers maintained that the change was not preceded by a 30-day notice period - meaning the new method is invalid. State lawyers countered that the injection protocol was part of the Department of Corrections' standing operating procedures that can be changed by the department commissioner without additional steps.
"We're pleased that the Supreme Court unanimously agreed with us," said Lauren Kane, a spokeswoman for the Attorney General of Georgia.
Kane didn't know how soon executions would resume in the wake of Monday's ruling.
V. Robert Denham Jr., an attorney for Hill, said he wasn't at liberty to discuss details of the case. Brian Kammer, another defense lawyer, also declined comment.
Hill's defense lawyers have also argued that he is mentally disabled and should be spared from death because the U.S. Supreme Court has ruled that executing the mentally disabled is cruel and unusual and Georgia law prohibits it.
State lawyers say Hill failed to prove that he's mentally disabled, and the state Supreme Court denied the inmate's request that it consider a challenge on that issue.
Hill was convicted in the 1990 beating death of fellow inmate Joseph Handspike. He "removed a two-by-six board embedded with nails that served as a sink leg in the prison bathroom and, as Handspike slept, pounded him in his head and chest with the board as onlooking prisoners pleaded with him to stop," Monday's ruling states.