The writer was learning about her family history on genealogy show Who Do You Think You Are? when it was revealed her ancestor, Frenchman Louis Volant, had joined the 16th Territorial Infantry Regiment in 1914.
Rowling was left stunned when a military historian revealed Volant was handed a Croix de Guerre (Cross of War) medal for his courage and bravery, after killing several German soldiers.
The author wept as she read out excerpts detailing the veteran's story, saying, "Oh my God... He killed several German soldiers. Oh my God. He was seriously injured in the arm and side by a shell, OK. Oh my goodness me... Amazing. That's absolutely amazing."
Rowling then burst into tears as the historian handed her a replica of Volant's Croix de Guerre. The speechless writer stared at the medal with her hands over her mouth.
But the author was in for a shock when she visited her ancestor's final resting place and found Volant had been buried in a communal grave. His remains were dug up and removed from the site in 1968 because of overcrowding at the cemetery - and none of his relatives were informed.
She raged, "I must be honest, part of me is very angry because he had family, and until relatively recently he had quite close family... The last of his children didn't die very long ago. And certainly my mother, who was so keen to find where he was buried, couldn't have had no idea what had happened. It's such a pity for a really extraordinary man."