Kate is currently touring the addiction charity's Hope House treatment centre in Clapham, south London.
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."
The Duchess of Cambridge will visit an addiction charity today to meet women recovering from alcohol and drug dependency. Kate will travel to Hope House treatment centre, in Clapham in her role as patron of Action on Addiction.
Each year a different charity, all with royal patrons, will receive fundraising support from 100 Women In Hedge Funds starting with Action on Addiction in 2013, then Harry's charity WellChild in 2014 and finally Kate's organisation the Art Room in 2015.
Kate's visit to the charity's treatment centre will be her first official appearance since pictures showing her on holiday in a bikini and with a very slight baby bump were published in a number of foreign magazines.