She appeared in Percy Jackson & the Olympians: The Lightning Thief as Aphrodite Girl and in Couples Retreat. In 2012, she starred in Project X as Alexis.
Knapp dated actor Ryan Phillippe in the summer of 2010, before splitting in September 2010. After the break-up, Knapp discovered that she was pregnant, and gave birth on July 1, 2011 to a daughter, Kailani Merizalde Phillippe Knapp.