The Pope called on then-Gov. Mel Carnahan to spare Mease and the governor complied. It is the only pure case of mercy I can recall in the "modern" era -- i.e., clemency coming without questions of guilt, severe mental illness and so on.
Curious, I dug up a review of a book that was written about this episode. Here are some pertinent paragraphs:
Almost Midnight: An American Story of Murder and Redemption is, in a sense, Mease's biography, and its singular achievement is to present Mease's story's full arc, allowing readers to understand how his story, like most other stories of death row prisoners, confounds the public's assumptions about these prisoners. He is, to paraphrase Sr. Helen Prejean, more than the worst thing he's done. Cuneo is at his best evoking the Ozark mountain culture in Missouri where Mease was raised. It's a curious combination of Pentecostal religious fervor and a backwoods way of life in which young men learn to shoot a gun at an early age, become profoundly suspicious of the law and are drawn to moonshining and cockfighting. While he was drawn to the Ozarks' outlaw culture, Mease, many believed, seemed destined to become a preacher.
Things changed, according to the author, after Mease's tour in Vietnam in 1967 when he became seriously involved with drinking and drugs. When he returned home, an increasingly paranoid and resentful Mease told a friend, "You know the Marines spent thousands of dollars teaching me to kill, and I still haven't killed anyone." Two marriages failed. When Mease began using crank methamphetamine in 1987, it prompted the radical, disturbing change in him that led him down the road to death row.
While on death row, Mease experienced a religious conversion that Cuneo and others believe was sincere. Mease became convinced God was his lawyer, and that he would be spared execution and eventually released from prison. A devout Pentecostal, Mease possessed core anti-Catholic prejudices. The pope was the last person Mease wanted to save him.
That's what happened, however, when Missouri serendipitously scheduled Mease's execution on the same day as a papal visit to St. Louis. A staunch death penalty proponent, Gov. Mel Carnahan was nonetheless embarrassed by the execution's timing and believed he had to pardon Mease when the pope pleaded personally at a papal Mass, "Governor, will you please have mercy on Mr. Mease?"
This extraordinary, unprecedented moment proved evanescent to death penalty opponents who hoped the pardon would slow the pace of executions in Missouri. There were six executions there in the six months after the pardon, and the pope's next plea for mercy from Rome was routinely dismissed.