Ward was born in Perth, Western Australia, as the second child of Gary and Claire Ward. Her older sister, Sophie Ward is also a model and she has younger twin brothers, Oscar and Henry Ward. She was educated at the Presbyterian Ladies' College, Perth and Shenton College.

At only sixteen, Ward was the youngest model to be listed by American Vogue as one of the nine "It Girls" in the modeling world. She has been in shows for designers such as Versace, Gucci, Chanel, Valentino, Alexander McQueen, and many more. Ward has also graced magazine covers such as Vogue, W, and TIME Magazine's Style and Design issue. In September 2004, at the age of 16, Ward became the youngest fashion model on the cover of the American edition of Vogue and was the first model to appear on the cover of Teen Vogue magazine. She is the first model since Kate Mossto be the face for Calvin Klein's perfume "Obsession Night for Men" along with Philadelphia male model, Drew Keilt.

To date Ward's clients have included Balenciaga, Burberry, Calvin Klein, Christian Dior, Dolce & Gabbana, Hermès, Jean-Paul Gaultier, Jil Sander, Karl Lagerfeld, Louis Vuitton, Prada, Rochas, Swarovski, Valentino and Yves Saint-Laurent, and, altogether, Ward has appeared on 24 covers of Vogue magazine worldwide.

British photographer Nick Knight stated, "Gemma is one of the very, very few models who look as though they come from another dimension." Fashion designer Lazaro Hernandez of Proenza Schouler told Teen Vogue in February 2006 that, "She looked like no one else at the time. She was this alien, and fashion is always looking for newness." Her look inspired a new generation of baby doll-like models such as Vlada Roslyakova, Heather Marks, and Jessica Stam.

In Allure magazine (October 2006), Ward is described as "the model of the moment." Allure's creative director Paul Cavaco knows why: "Gemma is ethereal, not earthy." Photographer Michael Thompson says, "She's an exotic blonde, the rarest of creatures."

Ward has long aspired to pursue acting and drama is a passion that she has held since she was a child. She has been cast in Australian director Elissa Down's upcoming film The Black Balloon also starring Toni Collette and Rhys Wakefield, as the girlfriend of a boy whose brother has autism. The Black Balloon had its world premiere at the Berlin International Film Festival in Germany in February 2008, where it received a Crystal Bear as the best feature-length film in the Generation 14plus category. She was also cast opposite Liv Tyler in the suspense thriller The Strangers as one of the three masked intruders, "Doll Face". She also appeared in the music video for "Daughters" by John Mayer

In July 2007, earning at an estimated total of $3 million in the past 12 months, Forbes named her tenth in the list of the World's 15 Top-Earning Supermodels.