Strickland questioned some of the eyewitness testimony against Kevin Keith, who was scheduled to die Sept. 15, and says he's bothered that other possible suspects weren't fully investigated.
Strickland said Keith has appeals available that could lead to a full reexamination of the crime, but he also acknowledged that might not happen.
Strickland said that despite evidence supporting Keith's guilt, "many legitimate questions have been raised regarding the evidence in support of the conviction and the investigation which led to it."
The "case is clearly one in which a full, fair analysis of all of the unanswered questions should be considered by a court," Strickland said. "Under these circumstances, I cannot allow Mr. Keith to be executed."
The move by the Democratic governor overrides last month's decision from his parole board, which unanimously recommended against clemency.
Strickland also overrode the parole board in 2008, when he commuted the death sentence of another inmate who claimed innocence.
Strickland made it clear he believes Keith is likely guilty of the 1994 slayings in Bucyrus, about 60 miles north of Columbus.
"There is evidence which links him to the crimes that, while circumstantial, is not otherwise well explained," Strickland said.
Police say Keith entered an apartment in Bucyrus on the evening of Feb. 13, 1994, and sprayed it with gunfire, killing Marichell Chatman, 24; her 4-year-old daughter, Marchae; and the Marichell's aunt, Linda Chatman, 39.
Three others were wounded, including siblings Quanita Reeves, 7, and Quentin Reeves, 4. Marichell Chatman was the sister of an undercover police informant whose efforts led to a drug raid weeks earlier during which Keith was arrested.
Keith's attorneys questioned eyewitness testimony that identified Keith as the shooter, provided evidence of an alternate suspect and lined up four alibi witnesses.
Keith's attorneys welcomed the decision while noting that their client, who had requested a pardon from Strickland, still needs a new trial to prove his innocence.
"The commutation to a life sentence does not lessen the need for justice to prevail," the attorneys said in a statement.
A message left with prosecutors in Crawford County was not immediately returned.
Keith's brother, Charles Keith, praised the decision, but said more had to be done.
"We're still in pursuit of the truth," said Charles Keith, of Canton. "Now we have to channel our energies and focus ourselves onto getting Kevin out completely."
Strickland spared John Spirko in 2008, citing a lack of physical evidence tying him to a 1982 murder. In 2009 he spared Jeffrey Hill, who killed his mother, Emma Hill, in a cocaine induced rage, after Hill's entire family asked for mercy. Strickland also spared Richard Nields in June, citing court decisions that questioned the appropriateness of a death sentence. Nields strangled his girlfriend during an argument.