As the family's primary wage earner, Mary's income as a midwife had enabled them to move into Forthlin Road in Allerton, where they lived until 1964.
Now in reply to a fan's question - picked from over 10,000 submitted to his website in a new monthly feature - the legendary rocker has revealed the depth of the heartache.
A fan from Brazil asked: "What would you do if you had a time machine" to which he simply replied: "Go back and spend time with my mum."
Previously, McCartney has spoken movingly of how his mother inspired one of his greatest songs Let It Be - written when he was living hard in 1968 with the growing pressures of The Beatles about to implode.