She initially planned to become a model, but was told that she was not tall enough at 5'5" to be a fashion model and should instead focus on acting in commercials. After taking commercial and acting classes, she fell in love with acting and realized that she preferred it over modeling. Greene graduated from high school early and moved to L.A. to pursue an acting career. Since then, she has appeared as a guest on such television programs, as Punk'd and Crossing Jordan.
Greene's big break came in 2008 when she was cast as Alice Cullen in Twilight, a film based on the novel of the same name by Stephenie Meyer. She reprised her role as Alice in the 2009 sequel to Twilight, The Twilight Saga: New Moon, 2010's The Twilight Saga: Eclipse, 2011's The Twilight Saga: Breaking Dawn Part 1, will reprise her role in 2012's The Twilight Saga: Breaking Dawn Part 2.