He was set to be executed Thursday in Huntsville, but one death row inmate decided not to wait.
Michael Johnson, 29, was to die for the 1995 shooting death of a convenience store clerk near Waco, but took his own life instead.
Johnson was found unconscious in his Polunsky Unit cell in Livingston early Thursday morning.
Prison officials say he had apparently used some sort of metal blade or razor to slash his jugular vein and arm.
"At 2:30 a.m. officers had seen him," Michelle Lyons with Department of Criminal Justice said. "He actually talked to some of the staff, and when they came back 15 minutes later he had already cut himself."
Johnson was taken to a Livingston hospital where he was pronounced dead roughly 15 hours before he was set to die for the 1995 shooting death of a convenience store clerk during a gas theft near Waco.
Johnson was scheduled to be executed at the Walls Unit in Huntsville.
He had visited his family Wednesday but denied them access to the execution and refused a last meal, signs officials say could have been interpreted as defiant, but staff maintain they never expected him to commit suicide.
Johnson had been on death watch with visual checks every 15 minutes and prison officials say he showed no suicidal signs.
"There had not been any indication that Mr. Johnson was contemplating suicide," Lyons said. "He me the day before with his family, he had met with reporters and certainly had not given any indication that he was even considering it."
Officials say Johnson had written words in blood in the cell where he fatally slashed himself, but they are not releasing details during the investigation.
They also say it's too soon to say if prison policy will change because of the incident, since this the first time since 1982 that a death row inmate committed suicide on the day of the execution.
The office of the Inspector General will investigate.
"They will be looking into it to see if there were any indicators and how he obtained the weapon and how he hid it," Lyons said.
Prison officials say more details will be released once the investigation is complete.
An autopsy will be conducted in Galveston County.