Jamie Larue Holmes had pleaded guilty to murder and arson. She will be eligible for parole when she's 75, prosecutor Jack McGaughey said.
Firefighters found the body of Brock Holmes, 18, inside the burning trailer near Nocona, northwest of Dallas, in June 2006. Police found his grandmother hiding in woods nearby.
She said she shot the teen in the back of the head while he was sleeping at his desk the morning after they had argued about his getting a job, McGaughey said. Holmes told authorities she planned to shoot herself but ran out of bullets.
Investigators never determined a motive, but McGaughey said Holmes indicated a conflict with her grandson that she did not want discussed during a trial.
"It seemed to be very important to her that her grandson's conduct toward her not be an issue brought out in court," the prosecutor said.