When Ambrosio was 12 years old, she partook in modeling classes, and then began modeling for Dilson Stein at age 15. Competing in Brazil's Elite Model Look competition started her modeling career in earnest. Her first notable modeling job was shooting the cover of Brazilian Elle magazine. Elite passed along some of her Polaroids to Guess which led her booking the Millennium GUESS? campaign. She has since appeared in advertising campaigns for Gucci, Dolce & Gabbana, Calvin Klein, Oscar de la Renta, Christian Dior, Escada, Fendi, Giorgio Armani, Guess, Emporio Armani, Moschino, Gap, Hugo Boss, Ralph Lauren, Saks Fifth Avenue, Macy's, Revlon, and the Pirelli Calendar. She has walked the catwalks for designers such as Prada, Chanel, Dolce & Gabbana, Givenchy, Christian Lacroix, Bottega Veneta, Escada, Tommy Hilfiger, Christian Dior, Marc Jacobs, Louis Vuitton, Balmain, Ralph Lauren, Halston, Vivienne Westwood, Giles Deacon, and Oscar de la Renta. She has appeared in numerous international magazine covers, including Cosmopolitan, Elle, GQ, Harper's Bazaar, Marie Claire, Ocean Drive, Vogue and was the only model to appear on the cover of Glamour in the United States in 2006.