The verdict means Stewart is spared the death penalty.
Stewart had admitted to shooting 11 people, killing eight of them, at the Pinelake Health and Rehabilitation Center in Carthage in 2009.
In trying to spare his life, Stewart's defense lawyers said the 47-year-old was essentially sleepwalking at the time due to taking a combination of prescription drugs.
Prosecutors spent 13 days presenting witness testimony and physical evidence to make a case for a first-degree murder conviction.
Assistant District Attorney Tiffany Bartholomew said Stewart knew what he was doing and was on a rampage looking for his estranged wife, who worked at the nursing home.
She said the shooting started in the parking lot when Stewart shot up his wife's car and he paraded through the nursing home, reloading every three shots as he fired at the elderly patients at point-blank range.
Residents Tessie Garner, 75; Lillian Dunn, 89; Jesse Musser, 88; Bessie Hedrick, 78; John Goldston, 78; Margaret Johnson, 89; Louise DeKler, 98; and nurse Jerry Avant, 39 - all died in the gunfire.
Defense attorney Jon Megerian said Ambien and other drugs in Stewart's system caused him to be in a zombie-like state of mind when he entered the nursing home.
Megerian said Stewart was depressed because his wife, Wanda Neal, had recently left him and he thought he was going to die from prostate cancer.
While jurors also found Stewart guilty of other assorted assault charges, they found him not guilty of attempted first-degree murder for shooting Carthage police officer Justin Garner.
Garner stopped Stewart's rampage by shooting him in the chest. Garner was wounded in the leg by a blast from Stewart's shotgun.
The trial now moves into the sentencing phase.