Seyfried was born in Allentown, Pennsylvania, on December 3, 1985. Her mother, Ann (née Sander), is an occupational therapist, and her father, Jack Seyfried, is a pharmacist. She is of mostly German descent, with smaller amounts of English, Scots-Irish, and Welsh ancestry. Seyfried graduated in 2003 from Allentown's William Allen High School. She has an older sister, Jennifer Seyfried, a musician in the Philadelphia organ-driven rock band Love City.
She began her career as a model when she was 11, then at 15 began an acting career, which initially included recurring parts on the soap operas As the World Turns and All My Children.
Her subsequent supporting roles were in independent films, such as the drama Nine Lives (2005) and the crime drama Alpha Dog (2006), she also had a recurring role in the UPN drama show Veronica Mars (2004–06). Between 2006 and 2011, she starred on the HBO drama series Big Love and appeared in the 2008 musical feature film Mamma Mia! and the 2018 sequel, Mamma Mia! Here We Go Again.
Her other appearances include leading roles in the black comedy horror film Jennifer's Body (2009), as a call girl in the erotic thriller Chloe (2009), the romantic drama-war film Dear John (2010), and the romantic drama Letters to Juliet (2010). She also stars in the dark fantasy historical romance Red Riding Hood (2011), the dystopian sci-fi thriller In Time (2011), the thriller Gone (2012), the musical drama film Les Misérables (2012), as the title porn actress in the biographical drama Lovelace (2013), and in the comedies A Million Ways to Die in the West (2014) and Ted 2 (2015).