At age 18, Knapp moved to Los Angeles. There in 2008 she hosted a popular internet video series titled 'Project Lore', a show about the videogame World of Warcraft, and also worked as a model before breaking into acting. She appeared in background roles in Percy Jackson & the Olympians: The Lightning Thief as Aphrodite Girl #9 and in Couples Retreat as a Dance Academy Girl. In 2012, Knapp appeared in her first major roles, playing Alexis in Project X, and as Stacie Conrad in the hit film Pitch Perfect playing a sex-crazed Barden Bella. She reprised her role in the latter's 2015 sequel, with a brief cameo in the 2017 sequel.