Ambrosio was born in Erechim, Brazil on April 11, 1981. Her parents, who are of Italian and Polish Brazilian descent, own a petrol station. Ambrósio was just eight years old when she decided that she wanted to be a model, after seeing a picture of top covergirl Karen Mulder in a magazine: "I wanted to be like her", she recalls. She enrolled at a modelling class at the age of 12 and at the age of 14 she was one of 20 finalists for an elite modelling competition in Brazil. Ambrosio was always insecure about her large ears and at eleven had cosmetic surgery to get her ears pinned back, though two years later she suffered complications. In 2006, she appeared on The Tyra Banks Show, and said that the surgery was a bad experience and has discouraged her from ever getting plastic surgery again.
When Alessandra 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.
In 2004, Ambrosio launched her line of swimwear called Alessandra Ambrosio by Sais, a division of Rosa Chá. It sold 10,000 units in only its first month on the market.
Ambrosio was the face for the UK company Next and starred in their first television campaign in 12 years. She is also signed to Storm Model Management in London. On the July, 2009 cover of Marie Claire, Ambrosio appeared in a spread with Sacha Baron Cohen to promote Baron Cohen's 2009 film Brüno.
Alessandra Ambrosio, became the face of Brazilian sportswear brand Colcci costarring with Ashton Kutcher in a steamy denim ad. She was ranked No. 5 on Forbes' The World's Top-Earning Models list, with an estimated earnings of $5 million over the past 12 months (2010–2011). In October 2011, Alessandra Ambrosio became Vogue's blogger, commenting on her wardrobe by posting one photo of herself each day for a month.
She appeared in the Victoria's Secret Fashion Show for the first time in 2000. By 2004, she was selected as the first spokesmodel for the Victoria's Secret "PINK" line and signed an exclusive "Angel" contract with the brand, being introduced as one at the "Angels Across America" Tour. In 2005, she walked the Victoria's Secret Fashion Show runway wearing lingerie made entirely out of candy. In the 2008 edition of the Fashion Show, she walked the runway three months after giving birth to her first child. She was chosen to open the 2010 show. The 2011 show saw her wearing the heaviest wings (30 lb) ever made for the show.
Alessandra Ambrosio has made several television appearances, notably a cameo as herself on HBO's Entourage along with Izabel Goulart; The Late Late Show with Craig Kilborn; Late Night with Conan O’Brien;Germanys next topmodel; as guest judge on Project Runway season 2: Team Lingerie and The Tyra Banks Show, where she was a guest twice. She appeared alongside TV personality, Regis Philbin in a worldwide commercial for Hummer H2. In 2006, she had a cameo on the box-office hit movie, Casino Royale appearing briefly as Tennis Girl #1.
Alessandra Ambrosio guest-starred on How I Met Your Mother (episode: "The Yips") on November 26, 2007 with her fellow Victoria's Secret Angels Adriana Lima, Selita Ebanks, Marisa Miller, Miranda Kerr, and Heidi Klum.
On August 24, 2008, Ambrosio and her fiancé, Californian businessman Jamie Mazur, had their first child, a daughter named Anja Louise Ambrosio Mazur, in Florianopolis, Brazil.
She is the National Ambassador for the National Multiple Sclerosis Society.
Ambrosio's Angel status has brought her to the public's attention, and she is often cited by the popular media as one of the world's sexiest women. As an Angel, she was chosen as one of People magazine's annual "100 Most Beautiful People in the World" in May 2007. Alessandra Ambrosio is among the world's top-earning models.