Michalka made her acting debut as Keely Teslow on the Disney Channel Original Series, Phil of the Future. She has also starred in the Disney Channel Original Movies Now You See It..., and Cow Belles, in which she appears opposite her sister. She had a leading role in the Walden Media film Bandslam, released in August 2009. She also played a role in the movie The Roommate. In 2004, Michalka was cast in the role of Keely Teslow on the Disney Channel's Phil of the Future. The series premiered on June 18, 2004.
In August 2006, Disney Channel ended the series after two seasons. Michalka also appeared in two Disney movies, Cow Belles and Now You See It.... Michalka made a guest appearance on CSI: NY, and in 2006, starred in an unsold Disney Channel pilot Haversham Hall which was not picked up. In 2007, Michalka starred alongside her sister AJ in the MTV television film Super Sweet 16: The Movie based on the MTV original series My Super Sweet 16. She played the role of the villainous popular mean girl Taylor Tiara. In 2009, she appeared in the Summit Entertainment musical comedy film Bandslam about a group of misfits who form a rock band. Despite failing at the box office, the film and Michalka's performance was well received among critics.
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 album, Insomniatic, hit stores, featuring the American top 20 hit single, "Potential Breakup Song". The two were to release a third album in early 2010, but it has been postponed after they left their label.
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.