The police officer's partner, retired Lt. Tom Kaiser, as well as relatives of the other victim witnessed Mitts die at the Southern Ohio Correctional Facility in Lucasville, The (Cleveland) Plain Dealer reported.
Mitts, 61, was sentenced to death in November 1994 after a firefight at his apartment complex left his neighbor, John Bryant, and Garfield Heights Police Sgt. Dennis Glivar dead.
Mitts' friend, Gary Hopkins, joined two ministers on the inmate's side of the witness room.
Last week, Gov. John Kasich denied Mitts' clemency request.
During the evening of Aug. 14, 1994, Mitts began his rampage by firing a laser-sighted round into Bryant's chest as Bryant, who was black, and his girlfriend, who was white, were returning home from a grocery store. Bryant died at the scene.
Mitts later opened fire on Glivar and Kaiser, shooting Glivar seven times, court documents indicated.
"We didn't even know he lived there [in the apartment complex]," Kaiser told The Plain Dealer Tuesday. "He was just waiting for us. Maybe he was looking through his peephole. He took us by surprise."
The battle ended hours later when a SWAT team shot tear gas into Mitts' apartment and subdued him.
Prosecutors argued Mitts' attack was racially motivated, but defense attorneys said Mitts killed Bryant to lure police to the scene.