Melbert Ray Ford, 49, died by lethal injection at 7:27 p.m. at the state prison in Jackson, according to department spokeswoman Kristen Stancil.
Earlier Wednesday, the U.S. Supreme Court denied Ford's motion to delay the execution. The Georgia Supreme Court had earlier voted unanimously to deny a similar appeal.
Ford's attorney Brian Mendelsohn had argued that the death penalty is disproportionate and amounts to cruel and unusual punishment because he's sat on death row for more than two decades. But state prosecutors countered that the arguments were a "ruse" and pointed to other rulings that repeatedly upheld the death sentence.
Prosecutors said Ford was seeking revenge when he killed Martha Chapman Matich and her niece Lisa Chapman in 1986. They said Ford began harassing her with phone calls after the couple broke up and soon was telling friends he wanted to kill her.
Ford was initially set to be executed on Feb. 23 but the execution was delayed by more than three months because a spot on the five-member state clemency panel hadn't been filled.
The execution was rescheduled for Wednesday after Gov. Sonny Perdue appointed a fifth member last month.
Authorities say anger drove Ford to plot his ex-girlfriend's death and that he was so infuriated that he tried to recruit several friends in a plot to drive him to the Newton County convenience store where she worked so he could rob it and then attack her.
Nobody would help him. Then he ran into Roger Turner, a 19-year-old who was out of a job and nearly penniless. Ford plied Turner with alcohol and the promise of thousands of dollars in cash, eventually persuading him to join the plot, according to court testimony.
The two drove in Turner's car to Chapman's Grocery shortly after it closed on March 6, 1986. Ford leapt out, shot away the lower half of the locked door and entered the store while Turner waited in the car. Turner later said he heard only screams and gunshots while waiting for Ford, who would soon emerge with a bag of money, according to court records.
When authorities arrived, they found Matich dead behind the counter, shot three times. Chapman was found sitting on a bucket in a bathroom, shot in the head and having convulsions. She died shortly after.
The two men were arrested the next day and Turner confessed to authorities. Ford, meanwhile, told investigators the shooting began after Matich pushed the alarm button, and that if he had worn a mask it would not have happened.
A Newton County jury convicted Ford and sentenced him to death after an October 1986 trial in which he claimed he was too drunk to know what was happening and that Turner was the one who entered the store and started firing.
Prosecutors dropped murder charges against Turner, a key witness in Ford's trial, and he was sentenced to 20 years in prison on robbery charges. Turner was released on parole after serving five years.
In several appeals, Ford argued that the jury failed to find any aggravating circumstances that would have justified a capital sentence. He also contended that prosecutors suppressed evidence about Turner's drug use the night of the killings and claimed his trial lawyer was ineffective.
The appeals were repeatedly denied by state and federal judges, and a petition to appeal the case to the U.S. Supreme Court was rejected on Jan. 25. The Georgia pardons board denied his appeal for clemency on Friday, and a county judge rejected a request to halt the execution on Tuesday.