She was born in 1877 not 1879
"England and Wales Birth Registration Index, 1837-2008," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:2X6F-5G5 : 1 October 2014), Mary Chinery, 1877; from "England & Wales Births, 1837-2006," database, findmypast (http://www.findmypast.com : 2012); citing Birth Registration, Medway, Kent, England, citing General Register Office, Southport, England
||Mother's Maiden Surname: |
|GRO Reference: 1877 J Quarter in MEDWAY Volume 02A Page 499|
Her surname was misspelled Chinery which is why you might have missed it.
She's probably the Mary Prentice who died in Q3 1962 at age 83. She would have actually been 85 but the 1939 Register has a birth year of 1879 as you've seen.
"England and Wales Death Registration Index 1837-2007," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:QVCK-RXWL : 4 September 2014), Mary Prentice, 1962; from "England & Wales Deaths, 1837-2006," database, findmypast (http://www.findmypast.com : 2012); citing Death, Rochford, Essex, England, General Register Office, Southport, England
As others have already said, there are already family trees for both her parents. Here is Mary Allen: https://www.familysearch.org/tree/person/sources/L82M-4WV