In 2005, within two months of moving to New York City, Decker auditioned for the Sports Illustrated Swimsuit Edition, in which she first appeared in 2006. She was again featured in the 2007 edition, and hosted the 2007 behind-the-scenes television special. She returned for a third consecutive time in 2008. Decker was chosen cover girl for the 2010 edition, shot on location in the Maldives islands. In 2007, she had a recurring feature on the CNN/SI website called "She Says, Z Says," along with Paul Zimmerman, discussing NFL football games of the week. Decker appeared in the 2007 made-for-television Lipshitz Saves the World. She worked for a period in Australia. She joined the cast of Sports New York to discuss the 2010 NCAA basketball tournament. In 2008, Decker rang the Stock Exchange bell with other Sports Illustrated models.

Decker is represented by Marilyn Agency.

Decker made a guest appearance on NBC's Chuck, where she played a model applying to be a "Buy More Girl." She made a guest appearance on USA Network's Royal Pains on July 30, 2009. Decker played swimsuit model Rachel, while Peter Jacobson played Alan, her nebbish husband.

On February 8, 2010, David Letterman announced on Late Show with David Letterman that Decker had been chosen as the 2010 Sports Illustrated Swimsuit covermodel. Calling her selection "the news of a lifetime," Decker admitted that her mother cried when she heard her daughter would grace the cover.