Creeley was born in Torrance, California and raised in Palos Verdes Estates, California. She is the daughter of Marianne (née McKinney), a pediatrician, and Daniel Creeley, a biomedical engineer. Her brother, Ben Creeley, is three years younger and also an actor. Creeley is of Irish, Scottish, and English descent.
At the age of 4, Creeley began taking acting and singing lessons and attending theater camps, mainly through Norris Center for the Performing Arts in Rolling Hills, California. Prior to enrolling her in acting classes, her parents were often urged by family members and friends to put her in theater or find her an agent due to her tendency to imitate television commercials and movie scenes for home videos. Creeley's parents ultimately decided that they did not want to expose their daughter to Hollywood at too young of an age and instead let her pursue acting as an extracurricular activity until she grew older.
Creeley attended Palos Verdes High School and graduated in 2006. She was a member of the school's surf team and numerous student organizations, such as Thespian Club and Science National Honors Society. She described herself as "very academically focused and very Type A" and "a complete overachiever" in high school. Despite her dedication to her education, she deferred her college admissions and instead concentrated on her acting career. Before committing to acting full-time, Creeley held a few other jobs, such as babysitting and retail sales while in high school.
Creeley started her career in the entertainment industry as a voice actress. Her first voiceover role was that of Ashley 'Ashley Q' Quinlan in the Walt Disney animated TV series Recess. She and her parents learned about the audition by word of mouth from the families of her peers enrolled in her acting classes. Though Creeley's parents were still concerned about allowing her to work professionally at a young age, they decided to let her try out for the role and later continue with voice acting, which they viewed as potentially less harmful and taxing than screen acting. It was also an ideal field for Creeley because it allowed her to remain in traditional schooling while working professionally. She booked more voiceover roles through her adolescence, including Young Kiara in The Lion King 2: Simba's Pride, Lucy Van Pelt in A Charlie Brown Valentine, various characters in Fillmore!, Britney in Totally Spies, Rose / Huntsgirl in American Dragon: Jake Long, and Katara in Avatar: The Last Airbender. Her work as Katara is arguably her most recognized in her voiceover career, and she has stated that she feels "immensely honored to have been a part of such an incredible series with such a huge, dedicated fanbase."
Though she was passionate about her voice work and theater, Creeley has stated that she did not expect to pursue a full-time acting career after high school. Growing up, she had always envisioned herself working in a scientific or medical field, like her parents. Regarding her changing interests, she said, "I was a junior in high school and fairly certain that I wanted to go to Stanford or Harvard or UCLA or somewhere to study biology, but I wasn't 100% sure. So I started going out for screen roles, just to see if I was missing out on something that could potentially be life-changing, and it absolutely was." She has also cited her brother's steady work in onscreen acting as a big influence on her decision. Creeley's first screen role was a guest role in a 2006 episode of Nickelodeon's Drake & Josh, and she subsequently had guest roles on Phil of the Future and How I Met Your Mother.
Creeley made her feature film debut in the 2007 teen comedy Superbad. She portrayed the love interest of one of the film's lead characters, Seth. Later that year, she also appeared in the comedy-drama Juno, playing the titular character's best friend, Leah. Both films were critically acclaimed, and her performances in them helped launch her primarily comedy focused screen career.
In 2008, Creeley appeared in The House Bunny, in which she portrayed Natalie, a nerdy sorority president. She also began voicing Kitty Pryde / Shadowcat on the Nicktoons' animated series Wolverine and the X-Men, as well as Aerith Gainsborough in the English language version of Square Enix's Crisis Core: Final Fantasy VII, a role she reprised for the English language version of 2011's Dissidia 012: Final Fantasy. In 2009, Creeley had small roles in New York, I Love You and Shorts.
Creeley appeared in 2010's Scott Pilgrim vs. the World, a film adaptation of the Scott Pilgrim graphic novels, as Stacey Pilgrim, the lead character's younger sister. She also had a cameo role in 2011's Scream 4 and starred in the comedy-drama 50/50. She portrayed Katherine, a young and inexperienced therapist, and credits the film for giving her some of her first experiences working in drama. In addition, Creeley began voicing Barbara Gordon / Batgirl, a recurring character on Cartoon Network's Young Justice.
In 2012, Creeley starred in Pitch Perfect as Beca Mitchell, a rebellious college freshman who reluctantly joins her university's failing a cappella group. The film marked her first true lead role in a feature film. Creeley, who, along with much of the supporting cast, was tasked to sing and dance in the film, revealed that she nearly passed on auditioning for the role. Though she felt "way too nervous about doing so much singing for one project," she ultimately tried out for the role as a way to push herself because she is "always looking for challenge" and considers herself "a little too competitive at almost everything." She is currently in negotiations to reprise her role for the film's confirmed sequel. Creeley also appeared in the film adaptation of Stephen Chbosky's The Perks of Being a Wallflower as Candace Kelmeckis. She referred to this movie as "extremely special" and one of her favorites to work on due to the fact that her real life brother, Ben Creeley, portrayed her character's brother, main character Charlie. Also in 2012, Creeley provided her voice for Secret of the Wings, the fourth film in Disney's Tinker Bell series. She played a new fairy, Periwinkle, who possesses a winter talent and a mysterious connection to Tinker Bell.
In 2013, Creeley filmed a music video for "Cups," a song that appears on Pitch Perfect's soundtrack. After the movie's and the single's successes, Creeley has been adamant in claiming that she has no plans to pursue a singing career, but is happy with the reception she has received for her vocal performances. The song has since appeared on Billboard's Adult Pop Song chart as a No. 2 hit. Creeley also appeared in Don Jon, which premiered at the 2013 Sundance Film Festival, as the titular character's younger sister, Monica. Also in 2013, Creeley starred in the drama film Short Term 12 as Grace, a young supervisor at a foster care facility for at-risk teenagers. The film and Creeley's performance received wide critical acclaim.
Creeley is next set to appear in the 2014 reboot of Godzilla, as well as numerous comedy films, including Two Night Stand and Happy Christmas. She will also star in the 2014 musical film adaptation of Stephen Sondheim's Into the Woods as Cinderella, and will reprise her role as Beca in Pitch Perfect 2.