The ruling from Vince Chhabria, the judge in a US district court in San Francisco, came after years of litigations on lawsuits claiming that the chemical glyphosate in Monsanto's weed killer spray has caused non-Hodgkin's lymphoma.
Johnson used Roundup for two years from 2012 as a groundskeeper for the Benicia school district near San Francisco, his lawyer Timothy Litzenburg told AFP.
Wisner said Johnson read the product's label and at one point contacted the company when he started to develop a rash.
Monsanto denies the allegations and in a statement said it would continue to defend the lawsuits with evidence proving there is "absolutely no connection between glyphosate and cancer".
"The single most relevant study, best study, study of human beings who, like Mr. Johnson, are licensed pesticide applicators concluded glyphosate is not associated with Non-Hodgkin's lymphoma, Mr. Johnson's cancer", said Lombardi.
In 2014, Johnson was diagnosed with non-Hodgkin's lymphoma, a cancer that affects white blood cells. But he said a reasonable jury could conclude, based on the findings of four experts he allowed, that glyphosate can cause cancer in humans.
The trial is expected to last about a month.
The stakes are high for Monsanto, which could reportedly face massive losses should a jury order it to pay out damages over the product.
The Monsanto brand may disappear soon, but claims that its most famous weed killer causes cancer will live on for years in court. Farmers in California, the most agriculturally productive state in the US, use it on more than 200 types of crops.
While evidence that glyphosate - the active ingredient in Roundup - can cause Hodgkin's lymphoma is "rather weak", the opinions of the three experts are not "junk science" that should be excluded from a trial, Chhabria ruled.
Monsanto is now facing hundreds of lawsuits, many of which were filed after that 2015 announcement. Homeowners use it on their lawns and gardens.
A flurry of lawsuits against Monsanto in federal and state courts followed, and California added glyphosate to its list of chemicals known to cause cancer.
Monsanto also pointed to sections of the opinion that questioned the plaintiffs' evidence. The report noted that science reviews by numerous other countries had reached the same conclusion.