Prosecutors told the panel in closing arguments that 38-year-old Daniel Ehrlick was a "ticking time bomb" on the day in 2009 when he killed Robert Manwill. The boy's body was found in an irrigation canal about a week after his disappearance triggered a massive search across the city of Boise.
Robert would have turned 10 earlier this month.
Ehrlick's sentencing was set for Sept. 2. He faces a maximum penalty of life in prison.
During his closing argument earlier in the day, Ada County Deputy Prosecutor Dan Dinger told jurors that Ehrlick panicked because Robert, who had been visiting his mother in Boise, was covered in bruises and about to go home to his father, who lived in New Plymouth.
"He was feeling that pressure, he was feeling that fear because that day was fast approaching," Dinger said.
Prosecutors claim Ehrlick feared Robert would tell about the abuse and he would lose his girlfriend's other son, an infant who had been previously removed from the home.
"It's been a long two years but today the system worked and we got some justice for my son," Charles Manwill, Robert's father, told reporters after the verdict was announced.
Jurors saw autopsy photos and heard recorded interviews in which Ehrlick told police he would hide Robert in a closet when social workers visited the apartment multiple times a week to check on the infant. Dinger told jurors that Ehrlick tortured the boy and hiding Robert in the closet was part of the abuse.
"He was preying on one of Robert's fears," Dinger said. "He knew that Robert hated to be alone."
The boy lived with his mother, Melissa Jenkins, and Ehrlick for about seven weeks and was bruised for much of that time, Dinger said.
The defense, however, rejected the notion that Ehrlick despised Robert and countered that Jenkins was responsible for much of the abuse the boy had suffered.
The prosecution examined more than 100 witnesses but did not present evidence that proved Ehrlick had killed the boy, said defense attorney Gus Cahill.
"He's told you under oath he didn't do this," Cahill said. "There's no confession, there's no admission."
Prosecutors claim Ehrlick, who weighed 277 pounds that summer, tortured the 50-pound boy in a pattern of escalating violence that ended with a fatal head injury.
"He'd been hurting him all summer long, but he hurt him one last night and he killed him," Dinger said.
In an agreement with prosecutors, Jenkins pleaded guilty in January to aiding in Robert's killing. She faces up to 25 years in prison at her August sentencing.