Emergency services were called to Park Close, Woodbury, Devon, on Thursday.
The woman was taken to Royal Devon and Exeter Hospital and then moved to Derriford Hospital, where she died on Saturday.
It thought she was washing her car when the it rolled forwards, Devon and Cornwall Police said.
A spokeswoman for the force said no formal identification had taken place but next of kin had been informed.
More on this story and others from Devon and Cornwall
A 79-year-old woman who was reportedly "crushed by her own car" as she washed it has died in hospital, police said.