Judge James Conlogue found the boy guilty after a hearing Friday in Cochise County Superior Court in the southern Arizona town of Bisbee. The boy is not being identified because he was charged as a juvenile.
Conlogue ruled that prosecutors had proved the boy acted intentionally and with premeditation when he shot Sara Madrid, 34, eight times on Aug. 1. The shooting happened after the boy had argued with his mother over his chores.
Madrid had left the family home after the argument, and the boy got a .22-caliber pistol from her bedroom closet, waited for her to return and then repeatedly shot her, according to court testimony.
Madrid's live-in boyfriend of 10 years, Alfonso Munoz, witnessed the shooting and said the boy gave him the empty gun afterward.
Munoz, who helped raise the boy, said he had taught the child how to use the weapon for emergencies and self-defense.
The boy's lawyer, Sanford Edleman, had argued that the boy did not intend to kill his mother but only wanted to get back at her for abusing him.
Police said the boy told them his mother yelled at him and slapped him.
Madrid's sister, Ernestine Huitron, testified Friday that Madrid did not want the boy to live with her, and that Madrid had a temper and yelled at him.
"He is a docile, sweet boy," Huitron said. "Sara said (the boy) was stupid and dumb."
Prosecutors wanted the boy tried as an adult, but the judge ruled the case should remain in juvenile court after a psychologist and psychiatrist testified that he suffered physical and verbal abuse from his mother and could be rehabilitated in the juvenile justice system.
Under Arizona law, the boy can only be held until he turns 18. Sentencing is set for Jan. 23.