When Ambrosio was 12 years old, she enrolled in modeling classes, and then began modeling for Dilson Stein at age 15. Winning Brazil's Elite Model Look started her modeling career in earnest. Her first notable modeling job was the cover of Brazilian Elle magazine. Elite passed along some of her Polaroids to Guess which led her booking the Millennium GUESS? campaign along with friend and fellow Brazilian Ana Beatriz Barros. She has since appeared in advertisements for brands like Revlon, Christian Dior, Giorgio Armani, and Ralph Lauren, as well as the Pirelli Calendar. She has walked the catwalks for designers such as Prada, Chanel, 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 the only model to appear on the cover of Glamour in the United States in 2006.