Decker was born in Kettering, Ohio, the daughter of Tessa (née Moore), a nurse, and Stephen Decker, a pacemaker salesman. She has one brother, Jordan, who is three years younger. She and her family moved to Middletown, Ohio, then to Matthews, North Carolina, a suburb of Charlotte. She is of German, English, Irish and Dutch ancestry.
She was discovered in a Charlotte shopping mall as a teenager. Decker began her modeling career as the face of Mauri Simone, a popular prom dress maker. She won the 2003 Model of the Year award at the Connections Model and Talent Convention. In a January 2011 interview with Self magazine, Decker stated that she suffered from an eating disorder and states, “My father did an intervention and got me to stop obsessing about everyone else and wrecking my body.”
Decker has been featured in pictorials in Teen Vogue, Cosmopolitan, FHM, and Glamour, and has modeled for The Gap, Intimissimi and Victoria’s Secret. She has appeared in music videos for Jimmy Buffett and the band 3 Doors Down. She is also an avid North Carolina Tar Heels basketball and Carolina Panthers fan.