Warren Hill (52) is reported to have an IQ of 70, putting him below the threshold for mental disability.
"There is no dispute among the experts that Mr Hill is mentally retarded," attorney Brian Kammer wrote in an appeal seeking leniency for his client.
"Because Mr Hill's execution would be a fundamental miscarriage of justice, this court must stay Mr Hill's imminent execution and vacate his death sentence," he added in another appeal.
After spending the last 21 years on death row for killing a fellow inmate, Hill was scheduled to die last July.
But the execution was put on hold for several months due to a change in how the killing was to be carried out; Hill is to be the first in the state to receive a single deadly dose of pentobarbital on Tuesday, instead of the previously standard three-drug lethal cocktail.
At the time, the state Supreme Court ruled unanimously to grant a stay of execution, saying it needed to investigate whether a lower court erred in determining that the change in execution protocol was legal under Georgia law.
The US Supreme Court ruled against the execution of prisoners with mental disabilities in 2002 but left each state with the authority to determine what constitutes as mental disability.
Strictest standard of any US state
Georgia has the strictest standard of any US state when it comes to determining this, with courts requiring "proof of retardation beyond a reasonable doubt" – a burden that some mental health professionals say is almost impossible to meet.
A Georgia court agreed with an appeal by Hill's lawyers that he should not be put to death, but it was overruled by a higher court in the state, which found that Hill could not prove his retardation beyond a reasonable doubt.
"While Georgia never contested Mr Hill's intellectual disability or IQ of 70, he was not able to meet the burden of proof," Eric Jacobson, the executive director of the Georgia Council on Developmental Disabilities wrote in an online editorial this week condemning the sentence.
But according to Kammer, the "uniform position of all experts in this case is that Mr Hill is mentally retarded."
Three doctors who had previously testified in favour of the state's position, wrote in a statement under oath that their initial evaluation was "extremely and unusually rushed" and "not conducive to an accurate assessment of Mr Hill's condition."
His girlfriend's murder
Hill was originally serving a life sentence in prison for murdering his girlfriend. While in prison, he bludgeoned a fellow inmate to death with a nail-studded board – the crime for which he received the death penalty.
He is now scheduled to die late on Tuesday in the Georgia town of Jackson by lethal injection but his defence team has submitted new appeals, including to the US Supreme Court.
Human rights groups, as well as the United Nations Office of the High Commissioner for Human Rights, condemned the impending execution and called on authorities to commute the sentence.
"The world community is again watching Georgia with great concern as it prepares to carry out another grotesque and unjust execution," wrote UN special rapporteur Christof Heyns in a recent editorial. "There is no sense and no honour in executing children, the insane and those who suffer from intellectual disability."
The execution would be the first in Georgia since the state carried out the 2011 death sentence of Troy Davis, despite evidence that many death penalty opponents believed exonerated him of his murder conviction. – AFP