Officers discovered the corpse in the women's shared home in the village of Mael-Carhaix in the northwestern Brittany region , deputy prosecutor Patrick Lewden told AFP, confirming local media reports.
"This person had not been seen since 2010," Lewden said, adding that an autopsy would be carried out to determine when she died.
The deceased woman was born in 1921. She lived alone with her 66-year-old daughter, who appeared to be suffering from mental health problems.
"Considering her mental state, (the daughter) had to be taken to a psychiatric hospital. She is very confused and could not be questioned," Lewden said.
Police searched the women's house after local authorities expressed concern about the mother's apparent disappearance.