The egg was given to nine-year-old Sybil Cook from Neath for Easter in 1939.
With war clouds looming, her uncle advised her to ration it and savour every morsel.
Despite loving chocolate, she did not just ration it, but dutifully kept and treasured it for her whole life, until she died in 2021.
After keeping it throughout WW2 - September 1, 1939 to September 2, 1945 - Ms Cook kept it uneaten and unbroken for another 76 years.
"Mum loved life and chocolate," said daughter Gill Bolter, 61, from Cardiff.
"With war looming her uncle said: 'You be careful with that my girl, there might not be any chocolate around soon'.
"He told her to ration it. Amazingly, she was so disciplined and respectful to her elders she never ate a single piece.
"When we asked mum how she'd managed to keep the egg for so long, she told us that having kept it all through the war it didn't seem right to eat it."
When Sybil died at the age of 91 in 2021, the Mary Mary Quite Contrary egg in blue and white paper remained intact, complete with a decorative garden scene of a little girl with a watering can.
The box still bears her name, Sybil Cook, written in pencil, and the year 1939.
Now, as a tribute to Sybil, her family have decided to part with the Easter egg to celebrate their mother's life and a special family memory.
The 84-year-old egg will be offered in Hansons Auctioneers' May 18 to 23 antiques and collectors auction, with a guide price of £600 to £800.
Ms Bolter added: "The egg was very precious to her. Having kept it safe through her childhood, she took it with her when she left home to get married in 1955 and for 60 years had it tucked away on a shelf in her bedroom.
"After we lost her, one of the care home nurses wrote a lovely tribute. She said: 'I would offer her a piece of chocolate after her evening medication. She'd always smile mischievously at me and say, 'Why not? It's the best medicine after all''.
"It was very hard on us all during Covid. Sadly, like thousands of other families, for 18 months we kept in touch with weekly visits outside the care home when rules allowed.
"But mum deteriorated in early 2021 and sadly passed away a week after her 91st birthday. I still get upset at the memory of us trying to sing happy birthday to her through a window.
"Her Easter egg brings back happy memories for us all including my daughter, her only grandchild.
"Mum loved antiques shows on TV and would have been thrilled to be part of this. It would be lovely if the egg went to a museum alongside mum's wartime memories."
Charles Hanson, owner of Hansons Auctioneers, said: "The story surrounding this Easter egg melted my heart. It's a wonderful reminder of wartime austerity, respectful obedience and a little girl who was so strict with herself she would not allow herself the tiniest nibble of her favourite treat.
"Preservation by not eating a chocolate egg for more than 80 years has made it valuable, although it's best before date is well gone by its edible value, its commercial value and its memories and nostalgia lives on.
"Food rationing lasted for 14 years in Britain, from 1940 until 1954 - nearly 10 years after the war ended. In 1946, when food was just as short as during the preceding years, bread was added to the ration and the sweet ration was halved.
"Sybil's egg is a reminder of those difficult days. She came from a generation who understood hardship. They learned to cherish and appreciate the smallest things. That's a very fine character trait to have."