He called the double Booker Prize-winning author's comments as "completely wrong", while Britain's patriotic press leapt to the defence of the pregnant Duchess of Cambridge.
But Mantel, 60, insisted that her comments had been distorted by the press and that she was supportive of the 31-year-old princess.
"I have absolutely nothing to apologise for," she said in an interview with BBC Radio 3's Night Waves programme.
"My lecture and the subsequent essay was actually supportive of the Royal Family and when I used those words about the Duchess of Cambridge, I was describing the perception of her which has been set up in the tabloid press.
"My speech ended with a plea to the press and to the media in general. I said 'back off and don't be brutes. Don't do to this young woman what you did to Diana'," she added.
In the essay, Mantel said Kate had neither the personality of William's late mother Diana, princess of Wales, nor the presence of historical heavyweight Anne Boleyn, who features heavily in Mantel's novel "Wolf Hall".
In a lecture given at the British Museum and reprinted in the London Review of Books literary journal, Mantel added that Kate appeared "machine-made" when she first emerged in public.
"Kate Middleton, as she was, appeared to have been designed by a committee and built by craftsmen, with a perfect plastic smile and the spindles of her limbs hand-turned and gloss-varnished," Mantel said.
She is the only woman to win the Booker Price twice, claiming the first in 2009 for "Wolf Hall", the opening part of her trilogy about Henry VIII's adviser Thomas Cromwell.
The second instalment, "Bring Up The Bodies", won the award last year.
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