The identity of Leonardo da Vinci’s mother has always been a mystery. Art historians have struggled to find information about the woman whose illegitimate son became the artistic genius who painted Mona Lisa. With only a possible first name – Caterina – there has been speculation that she was a peasant or even a slave from north Africa. Beyond that, there is little to go on.

Now, almost six centuries later, one of the world’s leading authorities on Leonardo has given a far fuller account of her story, piecing together her son’s world using previously overlooked documents.

According to Professor Martin Kemp, emeritus professor of art history at Oxford University, Leonardo’s mother was Caterina di Meo Lippi, a poor and vulnerable orphan, and only 15 when she was seduced by a lawyer. She had been living with her grandmother in a decrepit farmhouse, about a mile from Vinci in the Tuscan hills.

Kemp makes his claims in a book out next month, Mona Lisa: The People and the Painting, written with Dr Giuseppe Pallanti, an economist and art researcher. Newly unearthed documents also cast light on the portrait’s famed sitter, Lisa del Giocondo, and her husband, Francesco. Far from being a genteel Florentine silk merchant, as previously supposed, he was “a sharp operator”, trading in sugar from Madeira, leather from Ireland, property, money – and slaves, Kemp believes, judging from evidence of regular purchases of female slaves.

The professors’ research will also challenge the generally recognised site of Leonardo’s birth, on 15 April 1452 – the so-called Casa Natale in Anchiano, two miles from Vinci. Comprising two adjoining buildings, the Casa has become a place of pilgrimage for tourists, but documents suggest that art lovers have been visiting the wrong site. He believes the birthplace was more likely to have been the house of his paternal grandfather in Vinci, where Leonardo grew up.

Kemp said: “Leonardo is the subject of absolutely extraordinary public interest. So getting the reality of his life sorted out is a matter of great consequence.”

The new insights follow a trawl through 15th-century financial documents held within the archives of Vinci and Florence. They have proved a goldmine, having been apparently overlooked by other art historians.

Much of the new evidence comes from property taxation returns. Kemp said: “Tuscany set up a very early system of wealth tax and rates on the value of a property. So this is an absolutely fantastic record because everyone had to submit returns … Of course, everyone pleads poverty. They all say ‘this house is falling down’, but inspectors went round.

Professor Martin Kemp: ‘Caterina’s was a real sob story.’ Photograph: David Levenson/Getty

“In the case of Vinci, they verified that Caterina’s father, who seems to be pretty useless, had a rickety house which wasn’t lived in and they couldn’t tax him ... He had disappeared and then apparently died young. So Caterina’s was a real sob story.”

Caterina had an infant stepbrother, Papo, and her grandmother died shortly before 1451, leaving them with no assets or support, apart from an uncle with a “half-ruined” house and cattle.

According to Kemp, she was seduced by 25-year-old Ser Piero da Vinci, an ambitious lawyer working in Florence. Documents show that he took a break in July 1451, “exactly the right weeks for her impregnation”, Kemp said. “Nice, spring evening probably in the fields – and that was it.”

The lawyer himself was due to get married and Kemp believes that his family would have also provided Caterina with a modest dowry. This would explain how – with no means or possessions – she was married off quickly to Antonio di Piero Buti, a local farmer “from her own stratum of society”.

Leonardo was nurtured in the nearby house of his grandfather, Antonio da Vinci. Such arrangements were not uncommon then, Kemp said. Antonio’s 1457 tax return lists family members, including Ser Piero’s illegitimate son, as “born of him and of Caterina”.

Documents show that Caterina went on to have a second son and four daughters, only a few miles away from her first son. They reveal a further link between the lawyer and Caterina: he conducted a minor legal transaction for her husband.

Kemp hopes his work will put an end to “totally implausible myths” that have built up about Leonardo’s life. “What had become progressively attractive in the public domain,” he says, “was that Caterina was a slave, an African slave, or even an Oriental slave. Caterina was a name that tends to be given to slaves.”

Kemp acknowledged that the research would be “disconcerting” for the Vinci authorities, which have turned the Casa Natale into a tourist attraction. “When I first went there, there were just basically stone shells ... They’ve now got multimedia displays, car parks and have been cosmetically improved.”

His book pokes fun at historians who have tried to make their name with new theories about Mona Lisa – “that she is Leonardo in drag … that she is a ‘lady of the night’.” As he says, “the legends become more true than the truths.”

Why had so many significant documents been ignored until now?

“It never fails to amaze you that the obvious moves haven’t been made,” said Kemp. “Archives are not tackled because, in current academia, you need to get quick results rather than slogging through material with no guarantees of returns.”