The identity of the woman, buried in 1787 under the Barfüsser church in Basel, has always been a mystery, the Basler Zeitung reported. She is believed to be Johnson's great-great-great-great-great-great-great grandmother, according to researchers who had been testing her body since 2015and announced their findings on Thursday.
She was buried right in front of the altar, clearly well fed and was wearing good quality clothes, indicating she was just as wealthy as her distant grandson was destined to be. The high concentration of mercury in her body is what preserved it enough for it to be exhumed and studied.
Since 1975, when her mummified body was unearthed in a church in the Swiss city of Basel, she had been a scientific mystery - a well-preserved corpse buried with proofs of her wealth, who for lack of identification took on a distinguished title: the Lady from Barfüsser church.
Genealogists and molecular geneticists extracted DNA from the big toe of the body, that was discovered at the Barfuesser Kirche church in Basel in 1975.
DNA tests show Bischoff died of mercury poisoning, which explains why the remains were so well preserved - the poison acted as a preservative.
DNA samples from the mummy were compared with living descendants of the family to reveal the conclusive result.
Now the scientists and the historians were sure: the mummy was none other than Anna Catharina Bischoff.
Born in Basel in 1719, she spent most of her life in the French city of Strasbourg. A 99.8 per cent match was found, and the relationship to the Johnson family is confirmed by records of births, deaths and marriages. It's there, while caring for patients with syphilis, that it's believed she contracted the sexually transmitted disease.
One of her surviving children, Anna Katharina Gernler (1739 - 1776), married German aristocrat Christian Hubert Baron Pfeffel von Kriegelstein.
As for Bischoff, the researchers say that after her husband's death she returned to Basel, where she received the insidious treatment that ultimately led to her own death a few years later.
Together, they had a son, named Stanley Johnson, who is the father of Boris.