Hidden in the mass of prisoners was 20-year-old Hungarian Jew Edith Steiner, alongside her mother – the only two members of their family not sent to the gas chambers.
After they were saved Edith – known to friends as Eci – caught 23-year-old Mr Mackay's eye at a village hall dance to celebrate their liberation.
Mr Mackay, whose own wartime exploits in the London Scottish 1st Battalion included escaping from an Italian POW camp dressed as an enemy soldier, spotted Miss Steiner at the dance but was too shy to approach her.
He sent a friend over to ask if she would dance with him, but Miss Steiner insisted she would only dance with him if he plucked up the courage himself.
The couple, pictured aged 20 and 23, in 1944, when they first met after a dance at a village hall
He did, and what followed was a whirlwind romance that saw her and her mother whisked back to Mr Mackay's native Scotland.
The couple married on July 17, 1946, and have been 'wholly dedicated' to each other ever since.
John and Eci married in Scotland in July 1946, and owned a hotel in Pitlochry before retiring
With a family of two children, seven grandchildren and five great-grand-children, they worked as hoteliers before retiring to live at a care home in Broughty Ferry, Dundee.
According to the couple's daughter, Sharon Mackay, Edith Steiner and her mother survied six weeks in Auschwitz in June 1944.
Having come from the Székesfehérvár region of Hungary, Eci and her mother were the only members of their family who survived the ghastly selection process at Auschwitz.
While Edith and her mother were set to one side to work - itself usually a slow and lingering death - the other members of the family were sent straight to the gas chamber.
While at Auschwitz, Eci managed to make 'keep sakes' she has kept with her ever since, including a comb and a poem book.
As the war drew to a close, and the Soviets advanced rapidly from the east, the Nazis marched camp inmates towards Germany to help tackle the desperate shortage of workers in factories and mines.
John himself was captured at Tobruk in North Africa in the early part of the war. A soldier with the London Scottish, he escaped an Italian prisoner of war camp with his friend dressed as Italian solidiers.
Edith, 92, and John, 96, will celebrate their 71st Valentine's Day tomorrow with a party