Kass was born in Paide, Estonia and grew up in Paide, the capital of Järva County. When she was a little girl, she lived in a cramped apartment in Sika village with her mother, who worked on a farm. At the age of fourteen, she was 'discovered' in a Tallinn supermarket by an Italian modeling scout. Kass had wanted to compete for the Miss Estonia title, but an offer to fly to Milan, Italy, to start a modeling career seemed like a more glamorous and lucrative option. Kass officially moved to Paris at the age of 18. Soon after, 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 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 also appeared in Victoria's Secret shows. In addition, Kass has posed in advertisements for Calvin Klein, Chanel, Gucci, Donna Karan, Byblos, Dsquared2, Givenchy, Fendi, Max Mara, Valentino, Ralph Lauren, Emanuel Ungaro, Michael Kors, Versace, Yves Saint Laurent, Tommy Hilfiger, Revlon, Chloé, Christian Dior, Just Cavalli, Kenzo, Krizia, Mango, Guy Laroche, Narciso Rodriguez, Trend Les Copains, Sephora and also for General Motors. She is also recognized for her appearances in a The Gap denim advertising campaign. Kass was a spokesperson for Revlon cosmetics, Sephora, one of the world's 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?.Kass is well-known for her distinctive runway walk. A quirk of the Estonian supermodel is that while she is very tall, her feet are quite small (about an American size seven). It is said that she sometimes simply falls out of her shoes while walking. She starred at the Victoria's Secret fashion show 1999, and also walked in the non-consecutive years of 2000, 2002, 2003, and 2008 for the company. She walked with models such as Cindy Crawford and Adriana Lima.

• Täna öösel me ei maga (2004) .... Alis
  ... aka Set Point (International: English title)
  ... aka We Will Not Sleep Tonight (literal English title)
• Point&Shoot (2004) .... Carmen
• Welcome to America (2002) .... Prof. Kaia Hiiernum