During a lecture at the British Museum, the double Booker Prize winner criticised Kate as appearing to have been "gloss-varnished" with a perfect plastic smile in contrast to Princess Diana, who she described as awkward and emotionally incontinent.
She went on to suggest that the "painfully thin" Kate was selected for her role of princess because she posed no risk of showing any character.
Mantel's remarks were made two weeks ago during a lecture at the British Museum, organised by London Review of Books (LRB), a month after her latest novel Bring Up the Bodies won the Costa prize.
She said: "Kate seems to have been selected for her role of princess because she was irreproachable: as painfully thin as anyone could wish, without quirks, without oddities, without the risk of the emergence of character. She appears precision-made, machine-made, so different from Diana whose human awkwardness and emotional incontinence showed in her every gesture."
Mantel, whose latest novels are set in the Tudor court, said she saw Kate becoming a "jointed doll on which certain rags are hung".
She added: "In those days (Kate) was a shop-window mannequin, with no personality of her own, entirely defined by what she wore. These days she is a mother-to-be, and draped in another set of threadbare attributions. Once she gets over being sick, the press will find that she is radiant.
"They will find that this young woman's life until now was nothing, her only point and purpose being to give birth."
She also blasted Kate's first official portrait by Paul Emsley, unveiled in January, labelling her eyes "dead" and wearing "the strained smile of a woman who really wants to tell the painter to bu**er off".
During the lecture Mantel went on to question whether the monarchy is a "suitable institution for a grown-up nation" in a society which sacrifices royal ladies and allows them to be entertainment.
However, Nick Barton, who heads Action on Addiction, which Kate supports as patron, today defended her against Ms Mantel's attack.
He described her as an "intelligent" woman genuinely interested in the work of his organisation.
Kate will tour the addiction charity's Hope House treatment centre in Clapham, south London, later today - her first solo engagement since she presented the BBC Sports Personality of the Year award.
Asked for his reaction to Mantel's criticisms, Mr Barton said: "I don't think it's for me to comment on that kind of stuff. I speak of what I know - somebody who wants to help, is helpful and genuinely interested and is intelligent.
"I can only speak (of) what I know. I've met her several times and I found her to be engaging, I found her very natural, I found her actually genuinely interested in the subject.
"You can tell a lot from people's questions and she asks really good questions. They're not routine stuff, they're questions of someone who wants to learn. I find her very easy to deal with."
David Cameron also leapt to the defence of the Duchess of Cambridge, describing Ms Mantel's comments as "completely misguided and completely wrong".
Speaking in New Delhi during his visit to India, Mr Cameron said: "I think she writes great books, but I think what she's said about Kate Middleton is completely misguided and completely wrong."
The Prime Minister added: "What I've seen of Princess Kate at public events, at the Olympics and elsewhere is this is someone who's bright, who's engaging, who's a fantastic ambassador for Britain.
"We should be proud of that, rather than make these rather misguided remarks."