Mumford, the middle child of parents Tom Mumford and Nancy Smith, was born and raised in Olympia, Washington, where she was home-schooled through fifth grade. After homeschooling, she attended Nova Middle School. She later attended Annie Wright Schools in Tacoma, Washington and Capital High School in Olympia.[1] She has an older sister, Anna, and a younger brother, Kai. Inspired at age seven by a local production of South Pacific, Mumford performed in high school plays and at Olympia's Capital Playhouse.[1]
Mumford starred as Lena Landry on the ABC paranormal series The River in 2012[4] and on the Fox series Lone Star in 2010, in which she was cast during her first pilot season after college.[5][6] She was the female lead in the Blumhouse Productions feature Not Safe For Work for Universal Pictures,[7] and the female lead in the indy Drones alongside Matt O'Leary, directed by Rick Rosenthal.[8]
In February 2012, she was cast as the female lead in an ABC drama pilot titled Reckless.[9] She played Anastasia Steele's roommate, Kate Kavanagh, in the Fifty Shades of Grey film (2015), and reprised the role in the sequels Fifty Shades Darker and Fifty Shades Freed.[10]