Michalka was born in Torrance, California; she grew up in Seattle, Washington and Southern California, with her sister, Amanda. Her father, Mark, owns a contracting company, and her mother, Carrie, is a musician and performed with the Christian "JC Band". She has German, English, Irish, and Scottish ancestry.
In April 2010, The CW announced Michalka was cast in the main role of Marti Perkins on the cheerleading drama series Hellcats. The show premiered to September 8, 2010 to 3 million viewers. Despite a successful premiere, ratings failed to remain high with the series finale reaching a low 1.19 million viewers. In May 2011, The CW announced the show was not renewed for a second season. In 2010 and 2011 respectively, Michalka had major supporting roles in films Easy A and The Roommate. Michalka is set to star in the independent drama film Long Time Gone alongside Eva Longoria, Virginia Madsen and Amanda Crew. The film is set to be released in late 2013.
Michalka and her sister comprise the musical duo 78violet, and released their first album, Into the Rush, on August 16, 2005. In September 2006, they brought out a holiday album covering 9 tracks, along with two originals, titled Acoustic Hearts of Winter. In July 2007, their third album, Insomniatic, hit stores, featuring the American top 20 hit single, "Potential Breakup Song". The two were to release a fourth album in early 2010, but it has been postponed after they left their label. They release a new single entitled "Hothouse", on July 8, 2013, with a new album to follow.
Michalka has played the piano since she was five and started playing the guitar at the age of 13. She started acting when she was five years old, mostly in church play productions. She is a Christian and practices the faith.