The jury deliberated for 4 1/2 hours before finding Buck guilty on all nine counts he was facing.
Buck was convicted in Los Angeles federal court of two counts of distribution of controlled substances resulting in death, stemming from the deaths of Gemmel Moore in July 2017 and Timothy Dean in January 2019.
Those charges both carry 20-year mandatory minimums.
A sentencing hearing will be scheduled for a later date.
Buck was also convicted of enticing Moore and another man to travel to Los Angeles to engage in prostitution; knowingly and intentionally distributing methamphetamine; and using his West Hollywood apartment for the purpose of distributing narcotics such as methamphetamine, and the sedatives gamma hydroxybutyric acid (GHB) and clonazepam.
Prosecutors alleged during the trial that Buck caused the deaths of Moore and Dean as a result of his "fetish" for injecting men with increasing doses of methamphetamine until they became comatose.
The defense had countered that the victims had underlying medical conditions that ended their lives.
"He would find desolate, vulnerable victims and push meth on them over and over ... until they went unconscious,'' Assistant U.S. Attorney Lindsay Bailey said in her closing argument last week
However, Ludlow Creary II, one of Buck's attorneys, argued that his client actually did nothing more than enjoy party-and-play sessions involving drugs and sex with men he met online. It had nothing to do with Buck that two men who suffered from serious medical conditions would die at his apartment 18 months apart, the attorney said.
Overdose deaths from meth, other drugs up in LA County during pandemic
"This is a subculture, a lifestyle that may be shocking to some of us,'' Creary said during his summation. Everyone involved was there voluntarily.''
Buck, who has reportedly given more than $500,000 to mostly Democratic politicians and causes since 2000, declined to testify in his own defense.
Over the course of eight days, federal prosecutors put on more than 20 witnesses, including four men who told of smoking methamphetamine that Buck provided and then being pressured to allow the defendant to shoot them up with the drug.
One of the men testified that he passed out after being given several injections, and then forced himself to leave the apartment and go to a nearby gas station, where he called paramedics.
The defense called just one witness to the stand. Dr. Marvin Pietruszka, who runs a private autopsy service in the San Fernando Valley, testified Friday that he analyzed photos, slides and medical reports on the bodies of Moore and Dean, finding that both men had serious underlying medical conditions that caused their deaths. Methamphetamine, he told the jury, had nothing to do with either death.
Moore, Pietruszka said, died of complications from AIDS and pulmonary edema, while Dean died 18 months later, also in Buck's apartment, of alcohol poisoning and heart disease.
Prosecution witnesses, including a county medical examiner, testified that both men died from methamphetamine overdoses.
City News Service contributed to this report.