In 1999, at the age of 16, she appeared in the Victoria's Secret lingerie catalogue. The shoot caused controversy, as Victoria's Secret stated that they would not use girls younger than 18. Soon after the Victoria's Secret shoot, she was signed by Versace, Ralph Lauren and Valentino.
Fontana has been the face of a variety of advertising campaigns including Shiatzy Chen, Armani Jeans, Chanel, Colcci, Disritmia, Dolce and Gabbana, Emporio Armani,Enrique Martinez, Escada, H&M, Helena Rubinstein, Hugo Boss, Hussein Chalayan, Massimo Dutti, Mango M. Officer, Nicole Farhi, Oscar de la Renta, Peter Hahn, Philips, Ralph Lauren, Revlon, Roberto Cavalli, Rosa Chá, Saks Fifth Avenue, Tommy Hilfiger, Triton, Valentino, Versace, Victoria's Secret, Vivara and others. Photographed by Steven Meisel, she was presented on the September 2004 cover of American Vogue as one of the "Models of the Moment".
As a youth, Fontana attended Catholic School in the Portão district of Curitiba. She is often seen making the Sign of the Cross in footage. She is a non-practising Roman Catholic. She was married to model Álvaro Jacomossi but they divorced in 2004. Their son, Zion, was born in 2003. Fontana married actor and model Henri Castelli on December 10, 2005 and their son, Lucas, was born on October 23, 2006 in São Paulo. Fontana and Castelli divorced in 2007. She was engaged to Rohan Marley but the engagement ended in September 2012.