The state attorney general immediately appealed the 14-day delay to the U.S. Supreme Court, The Philadelphia Inquirer reported.
Hubert L. Michael had been scheduled to die by lethal injection at 7 p.m. at Rockview Prison in central Pennsylvania.
A three-judge appellate panel in Philadelphia ordered U.S. District Judge John Jones III to review his decision to deny a stay while allowing Michael to continue to appeal his sentence.
"Mr. Michael has suffered from debilitating mental conditions throughout his life," said Helen Marino, head of the death penalty unit in the federal public defenders office. "The Court of Appeals recognized that there are complicated issues involved in this case that should be carefully considered."
Pennsylvania has not carried out an execution since 1999. The three men put to death under a capital punishment law enacted in 1978 all waived appeals, effectively volunteering to be put to death.
Michael pleaded guilty in 1994 to the first-degree murder of Trista Eng, 16, in York County in south-central Pennsylvania. A judge sentenced him to death after he waived a penalty phase before a jury and initially filed no appeals.
In 2004, as his first execution date approached, Michael appealed the sentence. Prosecutors argued his 1994 waiver barred further appeals.