So they awarded him $289 million in damages -- mostly to punish the agricultural company Monsanto.
Dewayne Johnson's victory Friday could set a massive precedent for thousands of other cases claiming Monsato's famous herbicide causes non-Hodgkin's lymphoma.
Johnson's case was the first to go to trial because doctors said he was near death. And in California, dying plaintiffs can be granted expedited trials.
CNN reported last year that more than 800 patients were suing Monsanto, claiming Roundup gave them cancer.
Since then, hundreds more plaintiffs -- including cancer patients, their spouses or their estates -- have also sued Monsanto, making similar claims.
After three days of deliberations this week, the jury at the Superior Court of California in San Francisco awarded Johnson $250 million in punitive damages and about $39 million in compensatory damages.
It won't change the fact that Johnson's two sons might lose their dad soon. But it will help them live more comfortably, Johnson's attorney Timothy Litzenburg said.
Doctors weren't sure Johnson would live long enough to see his trial.
"He's going to live the rest of that time in extreme comfort," Litzenburg said.
After the verdict, Monsanto issued a statement saying it stands by the studies that suggest Roundup does not cause cancer.
"We will appeal this decision and continue to vigorously defend this product, which has a 40-year history of safe use and continues to be a vital, effective and safe tool for farmers and others," Monsanto Vice President Scott Partridge said.
But Litzenburg said an appeal would be costly for Monsanto, since the company would have to pay interest on the damages while the case is being appealed. That's about $25 million a year, he said.
Lesions on much of his body
Johnson, 46, applied Roundup weedkiller 20 to 30 times per year while working as a groundskeeper for a school district near San Francisco, his attorneys said.
He testified that during his work, he had two accidents in which he was soaked with the product. The first accident happened in 2012.
Two years later, in 2014, he was diagnosed with non-Hodgkin's lymphoma.
On bad days, Johnson is too crippled to speak. Lesions cover as much as 80% of his body.
Johnson had lesions on most of his body, a doctor said.
Litzenburg said the most heartbreaking part of Johnson's testimony was when the father of two described telling his sons that he had terminal cancer. Johnson's wife now works two 40-hour-per-week jobs to support the family, Litzenburg said.