"I am so flattered to join L'Oréal Paris," the 29-year-old model said in a statement. "Getting the chance to say 'You're Worth It' to women around the world is a great honour."
Lara is delighted to be L'Oréal's new spokesperson, but says it was something she "ever expected".
"It wasn't really anything I ever expected to happen in my career," she told Style.com. Talking about the rollcall of famous women to front campaigns for the brand previously, she added, "I never could really see myself in their line-up."
Speaking about their decision to hire Lara, a spokesperson for L'Oréal said, "Lara is a real modern icon. Her rebellious, edgy sensual beauty has made her so strong and successful. Yet, she remains incredibly genuine, true, sensitive and super fun.
"I love her iconoclastic style and the way she distinguished herself on every single fashion show she walked for. We are thrilled to welcome this exceptional woman and young mother to our brand."
Lara is married to Brit funnyman David Walliams. The couple welcomed their first baby into the world in May of this year.
David announced his wife's pregnancy on his Twitter page last year, writing, "Some wonderful news. My beautiful wife Lara is pregnant. We are having a baby! It is due next year and we both couldn't be happier. Dx."
David, 41, and Lara started dating in September 2009 and got married in May 2010.
Last year, Stone revealed she and her husband had been preparing for parenthood by caring for their pet dog Bert.
Speaking about their beloved mutt, she jokingly said, "We love him. A baby will probably never be as fun as Bert!"