The heavy metal rocker headed a campaign to find Sally Estabrook after she disappeared in October (14) during a camping trip in California.
The 75 year old, who suffered from Alzheimer's disease, was believed to have wandered away from the campsite where she was staying.
It has now emerged that a body found in the town of Julian, four miles (6.4 kilometres) away from where she disappeared, was identified as that of Estabrook on Thursday (27Nov14), the day of America's Thanksgiving holiday.
Officials at the County Medical Examiner's Office are to conduct an autopsy in a bid to establish how she died, according to Utsandiego.com.
Mustaine shared the sad news with fans in a post on his Facebook.com page on Thursday, writing, "Yesterday evening I received a call that a person had located a body and the investigators from SDSD Homicide Division described our beloved mother, Sally Estabrook. Thank you all for your prayers, your good thoughts, and positive vibes."
Mustaine's daughter, Electra, also posted a moving tribute to her grandmother, writing in a message on Instagram.com, "Nana... You are an angel now, but this is not new... You always have been. I'm so thankful for you. This Thanksgiving we will be celebrating your life... Rest in peace."
Estabrook was the mother of Mustaine's wife, Pam.