John Hill, who arrived in court in a wheelchair, had claimed that his loss of memory and other symptoms were due to chronic chemical poisoning from treating crops with the OP insecticide, Actellic D.
Mr Hill, 60, is seeking damages after being dismissed by his employer William Tomkins Ltd, of Peterborough.
But the 106-page written judgment by Mrs Justice Smith did not represent a total triumph for Mr Hill. The judge ruled that some of his continuing symptoms were due to other factors.
"My findings are in general that he has suffered ill-effects from his exposure to Actellic D in 1993, and some of his continuing symptoms are attributable to that exposure, but some of them are in part attributable to psychological factors and have, at times, been exaggerated by him and his wife in description to doctors - and have also on occasions been affected by the frequency with which he has been examined by doctors," she said.
William Tomkins Ltd admitted that it exposed Mr Hill to Actellic D and that he suffered an adverse reaction to it.
But it argued that the effects of the exposure were short-lived and that any long-term ill health from which he had suffered since 1993 had not been caused or contributed to by his exposure.
It is likely that the two sides will come to an agreement on damages.
Elizabeth Sigmund, co-ordinator of the OP Information Network, said: "This is much more than just a partial victory. The contention was that Mr Hill was neurotic and a hypochondriac. The judge has accepted that he has got real psychiatric damage."
Organophosphates have been blamed for inducing chronic fatigue, depression, panic attacks and memory loss in those who are exposed to them without adequate protection.
The most frequent complainants are farmers who have used the chemicals in sheep dips or as insecticides on crops. Hundreds of Gulf War veterans have claimed they were made ill by their exposure to OP sprays, used to kill desert pests.