Dewayne Lee Johnson filed the civil suit against the pesticide manufacturer.
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Johnson's attorneys say after two and-a-half years of spraying the pesticide, he was diagnosed with non-Hodgkins Lymphoma.
Attorney Brent Wisner told jurors they have the chance to change the world with a decision that would hold Monsanto accountable.
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"Today is their day of reckoning. Every single cancer risk that has been found had this moment where the science finally caught up, where they couldn't bury it anymore," Wisner told jurors.
Monsanto argues the evidence shows its product is not responsible.