Kass moved to Paris at the age of 18. She received exposure on the cover of magazines worldwide, such as Vogue (France) in 1997, ELLE (Australia), Image (UK), Madame Figaro, Numéro France, Vogue (France), and Vogue (US) in 1999.
In 1999, Kass modeled for top designers such as Marc Jacobs, Michael Kors, Calvin Klein, Ralph Lauren, Donna Karan, Dolce & Gabbana, Gucci, Prada, Versace, Fendi, Chanel, Chloé, Valentino, Louis Vuitton, and Givenchy. She appeared in Victoria's Secret shows.
Kass has posed in numerous advertisements for Calvin Klein, Chanel, Donna Karan, Givenchy, Fendi, Michael Kors, Versace, and for General Motors. She is recognized for her appearances in a The Gap denim advertising campaign. Kass was a spokesperson for Sephora, one of the leading cosmetics retailers, and the Christian Dior perfume, J'adore.
Kass was awarded by Vogue as "Model of the Year" at the 2000 VH1/Vogue Fashion Awards.
As of 2006, she has appeared in campaigns for Dsquared, Pedro del Hierro, Michael Kors and Chloé with Malgosia Bela. In 2007, Kass has got ten campaigns only for the spring/summer collection, including Versace, Michael Kors, Pucci, Jones New York, Salvatore Ferragamo, Gap, Peek & Cloppenburg, Bedat & Co, Time and Et Vous?.
She has now signed to be Max Factor newest spokesperson.
Kass is known for her distinctive runway walk. A quirk of the Estonian supermodel is that while she is tall, her feet are quite small (about an American size seven). It is said that she sometimes falls out of her shoes while walking.