Thomas Swan apparently hurled his British War Medal out of a train window soon after being presented with it in 1921 as he wanted to forget the horror of the trenches.
Mr Swan, a father-of-four, worked as a milkman in north London after the war and rarely talked about his experiences in northern France before he died aged 55 in 1935
He is thought to have been sickened by his military service as he had been deemed unfit to fight and ordered to bury thousands of dead servicemen.
The medal lay undisturbed until it was found in November 1999 by Bob Sheppard, who was out with his metal detector in a field close to the railway line in Betchworth, Surrey.
He cleaned it up and saw that it was inscribed as having belonged to Private Thomas Swan who served in the Queen’s Regiment.
Mr Sheppard, 62, of Crawley, West Sussex, then spent a fruitless 11 years trying to find any family of Mr Swan so he could give them the medal.