She joins names including designer Karl Lagerfeld in the magazine's list and is also named in its annual international best-dressed list for the fifth time.
Others on this year's list include Sherlock star Benedict Cumberbatch, actor Idris Elba and Downton Abbey's Michelle Dockery, who all make their debut in the ranks of the world-wide well-dressed.
Fellow actor Eddie Redmayne makes his second appearance on the list, as do husband-and-wife Colin and Livia Firth.
It also includes two Oscar winners in Cate Blanchett and 12 Years A Slave star Lupita Nyong'o.
The Duchess is not the only royal on the list, with Argentinian-born Queen Maxima of the Netherlands and the king of Bhutan also included.