Dale Hausner, dubbed the "Serial Shooter," was sentenced to die by lethal injection four years ago for gunning down four men and two women during a 14-month shooting spree. He and an accomplice were arrested in August 2006.
The Arizona Department of Corrections said Hausner was found unresponsive at the state prison in Florence, about 60 miles southeast of Phoenix, around noon on Wednesday, and was later pronounced dead at a local hospital.
No cause of death was given and an investigation was underway, the department said.
Hausner, a 40-year-old former janitor, was convicted of six murders and 74 other crimes committed during drive-by shootings of people and animals in 2005 and 2006.
An accomplice and former roommate, Samuel Dieteman, was sentenced to life in prison after he pleaded guilty to two murder charges and testified against Hausner during a six-month trial.
In a letter to the Arizona Supreme Court in July, Hausner said he wished to be "executed as soon as possible."
The court had just upheld his death sentence following a mandatory appeal, and in the letter, Hausner wrote that he did not want his lawyers to pursue any further appeal to the U.S. Supreme Court.
"They want to do all they can for all my appeals, and I appreciate their efforts, but it is my choice and I want to waive all the rest of my appeals," Hausner said. "I want to be served with my death warrant and to be executed as soon as possible."
(Reporting by Tim Gaynor; Editing by Cynthia Johnston and Stacey Joyce)