A California jury on Friday ordered the USA agrochemicals giant - which was taken over by Germany's Bayer in June - to pay almost $290 million in compensation to a groundskeeper diagnosed with cancer after he repeatedly used Monsanto's weed killer, Roundup.
The jury said that the herbicide's active ingredient, glyphosate, contributed to his cancer, and that the firm should have provided a label warning customers about the product's potential health hazards.
The German company, which owns agriculture giant Monsanto, says herbicides containing glyphosate are safe. The company says it will appeal the verdict.
Litzenburg and his attorneys have found more than 2,000 cases in states around the US, and another 400 have been filed in federal multidistrict litigation.
Records unsealed earlier by a federal court lent credence to Johnson's claims - internal company emails with regulators suggested Monsanto had ghostwritten research later attributed to academics.
A ruling in 2015 by the World Health Organization's worldwide agency for research on cancer (IARC) classified glyphosate as "probably carcinogenic to humans".
Johnson said he hoped his verdict would bolster the other cases.
"The jury got it wrong", Monsanto Vice-President Scott Partridge said outside the courthouse in San Francisco.
Environmentalists say the weedkiller is linked to cancer, although the claim is strongly denied by manufacturers and the European Union has approved the chemical for use. Monsanto has denied a link between the active ingredient in Roundup - glyphosate - and cancer, saying hundreds of studies have established that glyphosate is safe.
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He was first diagnosed with non-Hodgkin's lymphoma - a cancer that starts in the white blood cells - in August 2014, having previously sprayed hundreds of gallons of Roundup.
His lawyers said he regularly applied Monsanto's weed killer up to 30 times per year at his job with the San Francisco Bay Area school district.
The court in San Francisco found that the manufacturer, Monsanto, had failed to warn Johnson and other consumers of the cancer risks posed by its weedkillers, including Roundup, which is on sale in Britain.
Roundup is widely available in British garden centres and is thought to be the most commonly used weedkiller in the world.
In France, a leading anti-Monsanto campaigner told AFP that the California ruling would strengthen the resolve of those doing battle with the agrochemicals giant across the world.
The jury said that the packaging of the product should have indicated the risks of using it.
Partridge, meanwhile, announced that Monsanto had no intention of settling the slew of similar cases in the legal queue.
After labeling glyphosate a "possible human carcinogen" in 1985, the US Environmental Protection Agency (EPA) reversed its position on the chemical in 1991.
A spokesperson for Wilko said it was not reviewing Roundup products, but added that it was: "watching the situation carefully and will respond appropriately".