A Superior Court jury convicted 63-year-old Claude Foulk of 31 of 35 counts of sex crimes, including lewd and lascivious acts on a child and sodomy by use of force.
Prosecutors say another 11 men also came forward to claim Foulk molested them as children dating back to 1965, but only the son's case could be prosecuted because of the statute of limitations.
Foulk was fired from his post at Napa State Hospital after his arrest last year.
He could face a maximum sentence of up to 248 years in prison when he is sentenced Feb. 23.
During an emotional weeklong trial, five now-grown men testified that Foulk abused them for years. They recalled how the man they knew as an uncle and foster father bought them pizza and took them up to a mountain cabin before forcing them to engage in sex acts.
One of Foulk's two adopted sons told jurors Foulk abused him from the time he was 9 years old until he was 21, telling him it was how a man shows love.
The Associated Press is not naming the witnesses because it has a policy not to identify alleged victims of sexual abuse.
One of the men said he was overjoyed about the verdict.
"My heart was beating so fast right before she read it," he said. "He'll rot in jail for the rest of his life and that makes me feel so good."
Foulk, who watched the testimony with a blank stare, took the stand in his own defense and denied the allegations.
Foulk worked as a nurse, obtained a master's in business administration and held previous state jobs before working at Napa State Hospital. He was a foster parent to two boys and adopted two sons.
Prosecutor Danette Gomez argued that Foulk used the foster care system to acquire boys to meet his insatiable sexual appetite, knowing they had no parents to turn to.
She said the years of horrific abuse led the boys to turn to alcohol and drugs and have trouble forming lasting relationships.
Foulk's attorney Richard Poland said Foulk's son, whose testimony was the heart of the case, had a history of lying. He also argued there was a lack of physical evidence, noting there were no medical reports or photographs to back up the allegations.
The investigation into Foulk was sparked when someone reported sexual abuse to police after learning Foulk was head of Napa State Hospital.