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 Armani Jeans, Chanel, Colcci, Disritmia, Dolce and Gabbana, Emporio Armani, Enrique Martinez, Escada, H&M, Helena Rubinstein, Hugo Boss, Hussein Chalayan, Massimo Dutti, M. Officer, Nicole Farhi, Oscar de la Renta, Peter Hahn, Philips, Ralph Lauren, Revlon, Roberto Cavalli, Rosa Chá, Saks Fifth Avenue, Tommy Hilfiger, Triton[disambiguation needed], Valentino, Versace, Victoria's Secret, Vivara and others.In 2008 casting agent James Scully, who is responsible to pick which model is to score a spot on top runways, says in regards to Fontana:''There is something almost saintly about her — if you told me she could heal sick children, I would be a believer! Yet on the other side of the coin she has a body better than all the Victoria's Secret girls combined. And one of the kindest models I've ever worked with. Glides down a runway like a panther. And the most beautiful blue eyes in the biz''
Isabeli is the current face of Viktor & Rolf's fragrance Flowerbomb.
In 2008, she made her debut on the Forbes The World's 15 Top-Earning Models list at place 11, having earned $3 million.As a youth Isabeli attended the Church of Jesus Christ of Latter-day Saints in the Portão district of Curitiba. 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 are separated now and are in the process of divorcing.