The case of school groundskeeper Dewayne Johnson was the first lawsuit to go to trial alleging glyphosate causes cancer.
The court heard Mr Johnson used the weedkiller in large amounts while working in the San Francisco Bay Area school district.
Furthermore, the jury found Monsanto officials acted with "malice and oppression" in their selling of the product despite its risks being known.
Last year, the CBS 11 I-Team sat down with Monsanto's vice president, Scott Partridge, at the company's headquarters in St. Louis.
The jury at San Francisco's Superior Court of California deliberated for three days before finding that Monsanto had failed to warn Johnson and other consumers of the cancer risks posed by its weed killers.
He is among more than 5,000 similar plaintiffs across the US.More news: Paul Pogba posts cryptic message on Instagram after Manchester United win
The lawsuit is the first to accuse the product of causing cancer, but observers say a Monsanto defeat likely opens the door to thousands of other claims against the company, which was recently acquired by Germany's Bayer. The use of Roundup and other glyphosate chemicals is gaining traction with farmers opting for herbicide-tolerant (HT) Bt cottonseeds. "You did something wrong and now you have to pay", said Brent Wisner, Johnson's lead trial lawyer. He worked at the time as a groundskeeper for the school district in Benicia, a suburb north of San Francisco. During that time, he mixed and sprayed hundreds of gallons of the chemical.
The maker of Roundup weedkiller has insisted British consumers are safe to use the product after a U.S. jury backed a groundsman's claim the pesticide contributed to his terminal cancer. "The company chose corporate profit and greed above humanity". Although not involved in the just-concluded trial, the firm represents more than 100 clients now with similar claims.
There is conflicted evidence linking glyphosate and cancer.
However, the US Environmental Protection Agency (EPA) insists it is safe when used carefully.
Campaigners are now calling for a review of pesticide regulations in the United Kingdom after the case, saying that glyphosate poses a risk to public health, soils and the environment.
In India, absence of long term studies regarding the effects of glyphosate use has made the task of regulation more hard.More news: The View From London: We're In A Full-Scale China Trade War
"This should send a strong message to the boardroom of Monsanto", Kennedy added.
Mr Johnson's attorneys sought and won $US39 million in compensatory damages and $US250 million of the $US373 million they wanted in punitive damages. "We will do everything to make sure Mr. Johnson and his family share in that award". He later made a call on his cellphone.
Monsanto pushed back on the verdict, vowing to appeal the case. The latest annual report of Monsanto's India unit states that its brand Roundup maintains market leadership with sales of around Rs 185.66 crore, which is 28 percent of its net sales of Rs 667.44 crore in FY18.Roundup is primarily used in agriculture and places like roadsides, public spaces, railway tracks and gardens.
To each of those questions, the jury's response was, "Yes".
"Was Monsanto's failure to warn a substantial factor in causing harm to Johnson?"More news: Best meteor shower of 2018 visible over Middle TN this weekend