The identity of the woman, buried in 1787 under the Barfüsser church in Basel, has always been a mystery, the Basler Zeitung reported. Boris said his grandmother had spoken about her aristocratic ancestors, but nobody in the family believed her claims.
While there was no gravestone to identify her, and her coffin was unmarked, she was buried in front of the altar, which is a symbol of great importance, and was wearing good quality clothes.
Highly toxic mercury was used as a treatment for syphilis but was more often a kill than a cure and scientists say it was this that preserved her body.
Scientists in Basel have been working to identify the body since it was uncovered in 1975, during the renovation of the city's Barfüsser Church.
It only became clear previous year, based on newly discovered archives, that the mummy had been uncovered once before, in 1843.
Her identity was established with 99.8 percent certainty after scientists extracted DNA from her big toe and compared it with samples from her descendants, The Guardian reported.
Archive records emerged which showed she had been discovered before in 1843 and they led historians to believe she was a member of the established Bischoffs family.
The research confirmed the woman was Anna Catharina.
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They learned she had seven children, but only two survived childhood.
Born in Basel in 1719, she spent most of her life in the French city of Strasbourg.
Although she is referred to as a mummy, she had an ordinary Christian burial, and her body wasn't intentionally kept intact, according to the Business Insider.
She is likely to have contracted syphilis while caring for patients affected by the sexually transmitted disease.
On the death of her husband she returned to her home town of Basel, and apparently underwent rigorous mercury treatment in the hope of a cure.
Mr Johnson has yet to comment on the discovery of his long lost relative, but he is aware of the von Pfeffel connection, having once told the BBC programme Who Do You Think You Are? that they were "posh toffs".