At 13, she was a finalist of the Elite Model Look in 1996. She moved to Milan, Italy to start her modeling career in the following year. When she appeared in the Victoria Secret lingerie catalogue at the age of 16, she stirred a controversy for being below 18. She was later on signed by Versace, Ralph Lauren and Valentino. Isabeli has appeared in various magazines such as Sports Illustrated Swimsuit Issue, ELLE, Vougue, Marie Claire, Harper's Bazaar, among others. Fontana has been endorsing the likes of Armani Jeans, Dolce and Gabbana, Chanel, Escada, H & M, Helena Rubinstein, Oscar dela Renta and other prestigious labels. She has also appeared in the famous Pirelli Calendar photographed by Karl Lagerfeld.