Jeffery Wood, who had been scheduled to be executed next Wednesday, was sitting in a pickup truck outside a gas station in January 1996 while his friend robbed the convenience store and shot the clerk inside.
Wood, who turned 43 on Friday, was sentenced to death under a state law that says anyone involved in a criminal plot resulting in death is equally responsible regardless of actual involvement or intent.
His lawyers argued that Wood's due process had been violated by false testimony and scientific evidence.
The Texas Court of Criminal Appeals agreed, basing the stay on the fact that an expert psychiatrist who provided his opinion of Wood's mental capacity was later discredited.
"The court did the right thing by staying Mr Wood's execution," his attorney Jared Tyler said in a statement.
Wood, who is said to have the IQ of a child, was unaware that his friend, Daniel Reneau -- whom he had met just two months earlier -- would carry a firearm into the convenience store, his supporters say.
Wood sat in the pickup truck outside the gas station in the city of Kerrville while Reneau went inside to rob a safe.
Although Reneau had anticipated an unarmed robbery, the plan went awry when he shot the store employee in the head after he refused to comply.
Hearing the gun go off, Wood rushed into the store to find a blood-soaked scene. He helped Reneau remove a video surveillance recorder before the two men fled, taking the safe and a cash box.
They were arrested the following day, quickly identified by witnesses.
"Justice is not served by executing Mr Wood, who was outside the building when it happened and who had no criminal history," Tyler said.
Wood's supporters waged an all-out campaign to stay the execution in recent days and several dozen evangelical leaders also wrote to Texas Governor Greg Abbott demanding clemency.
"I have never seen an execution in the United States with this low of a level of culpability as Mr Wood has," his defense attorney Kate Black told AFP.
