She has worked as the face of brands such as Chanel, Versace, Yves Saint Laurent, Gucci, Valentino, Calvin Klein, Ralph Lauren, Christian Dior, Givenchy, and, for ten consecutive years, Michael Kors.

The American edition of Vogue declared her and Gisele Bündchen the two supermodels of the era on its January 2000 cover.

Her first venture into the modeling world was unsuccessful, and Kass left Milan after a short time.

Her career was launched by Anna Wintour,[7] and by 1999, Kass was walking the runway for numerous and diverse top designers, such as Marc Jacobs, Michael Kors, Calvin Klein, Ralph Lauren, Donna Karan, and Dolce & Gabbana; and posing in advertisements for designers and brands, like Calvin Klein, Chanel, Gucci, Donna Karan, Dsquared2, Versace, Givenchy, Fendi, Max Mara, Valentino, Ralph Lauren, Narciso Rodriguez, and even General Motors.

[8] She also appeared in a Gap denim advertising campaign, and was a spokesperson for Revlon cosmetics, Sephora, and the Christian Dior perfume, J'adore.

[20] In 2004, Kass played the female lead in the Estonian murder-mystery film Täna öösel me ei maga ("We Won't Sleep Tonight"; titled "Set Point" in the international release).

[21] She also had a cameo appearance in the film Zoolander, where she played herself opening the Derelicte fashion show.