Cody Posey was convicted of murder in the deaths of his stepmother and stepsister and voluntary manslaughter in the death of his father, who worked as a foreman on Donaldson's ranch in southern New Mexico.
Posey also was convicted of four counts of evidence tampering. He hung his head and wept as the verdicts were read.
Posey could face life in prison on the first-degree murder charge in the death of his stepsister. State District Judge Waylon Counts said a sentencing hearing would be scheduled within 30 days.
The teen was arrested a few days after the bodies of his father and stepmother, Paul and Tryone Posey, and his 13-year-old stepsister, Mary Lee Schmid, were found hidden in a manure pile on Donaldson's Chavez Canyon Ranch in July 2004.
Donaldson was the trial's first witness, taking the stand Jan. 17 to describe finding a bloody scene upon returning from a trip.
The jury, which heard three weeks of testimony, began deliberating Monday.
Verlin Posey, brother of Paul Posey, told reporters outside the courthouse he was pleased with the verdict but disappointed the teen was not convicted of first-degree murder in his brother's death.
"I didn't like it, but I can live with it," he said.
Prosecutor Janice Schryer claimed that Posey's parents were loving, but said the youth did not like ranch life and "took the opportunity to relieve himself of those expectations and that life."
Defense attorney Gary Mitchell argued that Posey, who was 14 at the time of the slayings, was abused by his parents.
He described years of physical and psychological abuse, and said the flashpoint to the murders came when Paul Posey burned the boy with a metal welding rod after the teen allegedly refused to have sex with his stepmother the night before the slayings.
But prosecutors contended the teen planned the slayings, then lied and tried to turn suspicion away from himself. Prosecution witnesses also testified they never saw a mark on Posey's body — not a scrape, bruise or black eye.
Ellen Brust, whose son was married to Cody's biological mother, spent two weeks attending the trial and offered her support for the boy.
"I know we'll be back and Cody will be vindicated," Brust said.