In his will, he bequeathed the house to the town, specifying that he wanted it to be made a museum a century after his death.
Although he left no orders to have it sealed, the mansion was left practically untouched all those years, its eerie calm even unbroken by the occupying German forces of the Second World War.
"It was very strange, the house became a sort of urban myth," said assistant curator Maud Leyoudec. "People didn't know what was in this house and had fantasies."
As the centenary of Mr Mantin's death approached, the town was spurred to act on his bequest as it faced losing the building to a surviving heir.
Under French law, Mr Mantin's great-niece Isabelle de Chavagnac could claim it back 100 years on.
When they opened up the house, experts found it in a "musty and awful" condition, with "insects everywhere".
There were no skeletons but a host of untouched treasures, including rich tapestries, extremely rare gilded leather wall coverings and contemporary artworks.
Mr Mantin had also incorporated a host of cutting-edge appliances hardly seen at the time, including electricity, a flushing lavatory, a fully-plumbed roll-top bath with overhead shower and towel-warming cupboard.
He had a museum of natural history curiosities, including two stuffed frogs fighting a duel and a rat playing a violin.
One room covered in pink "amour" wallpaper was dedicated to the respectable gentleman's concealed and scandalous 20-year relationship with his married mistress. A winking woman's face over the fireplace was another wry reference to the affair.
Thanks to a £2.9 million refurbishment funded by local authorities, the mansion has been fully restored and is now open to the public.
"Mantin was obsessed with the passing of time, and death," said Miss Leyoudec. "He wanted the house to remain unchanged, like a time-capsule for future generations, so they would know how a bourgeois gentleman lived at the turn of the 20th century."
There was a more selfish motive, though, she added.
"He wanted to survive. Now everyone in Moulins knows the name of Mantin."