Michalka was born in Torrance, California. She grew up in Seattle, Washington and Southern California, with her younger sister, Amanda "AJ", who is also an actress and musician. Her father, Mark, owns a contracting company, and her mother, Carrie, is a musician and performed with the Christian "JC Band".
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. She has also starred in the Disney Channel original movie Now You See It..., and appeared in the TV movie Cow Belles opposite her sister, AJ Michalka. Michalka and her sister starred in several projects produced by the Disney Channel and MTV. In 2007, they appeared in the MTV film Super Sweet 16: The Movie based on the MTV series My Super Sweet 16. In August 2009, she co-starred in the film, Walden Media film Bandslam. In April 2010, The CW announced Michalka was cast in the main role of Marti Perkins on the cheerleading drama series Hellcats. Despite a successful premiere, ratings dropped, and in May 2011, The CW announced the show was not renewed for a second season. In 2011, Michalka made a guest appearance on CSI: NY. In 2010 and 2011 respectively, Michalka had major supporting roles in films Easy A and The Roommate. Michalka appeared in the independent drama film, Crazy Kind of Love, alongside Eva Longoria, Virginia Madsen and Amanda Crew. The film was released in July 2013.
In 2013, Michalka participated in a multi-episode role on the sitcom Two and a Half Men. In July 2014, she and her sister, filmed in the independent drama, Weepah Way for Now, in Laurel Canyon, California. Michalka participated in the casting process and production for the film. The movie was written and directed by her husband, Stephen Ringer. The film later premiered at the Los Angeles Film Festival on June 16, 2015. Since March 2015, Michalka had a recurring role on the CW series iZombie for the first and second season and became a regular cast member for season three. On April 19, 2016, she was announced as a cast member for the upcoming comedy film The Lears, an adaptation based on the Shakespearean play King Lear.
Michalka and her sister, AJ Michalka, formed the musical duo Aly & AJ in 2004. They released their first album, Into the Rush, on August 16, 2005. In September 2006, they released a holiday album, Acoustic Hearts of Winter, featuring cover songs and composed two original songs. In July 2007, their third album, Insomniatic, was released, featuring the Billboard top 20 hit single, "Potential Breakup Song". The duo changed their name to 78violet and were set to release a fourth album in early 2010, but the project never materialized following their departure from Hollywood Records. Several years later, in the summer of 2013, they returned with new music, releasing their first single, "Hothouse", on July 8, 2013. In early December 2015, they confirmed they returned to their original name, "Aly & AJ". In April 2016, they confirmed plans to release new music for an upcoming studio album. On June 2, 2017, they announced a new single, titled "Take Me". It serves as the lead single from their EP, planned to be released on July 14, 2017.
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 in church play productions. Her parents are divorced. In 2012, Michalka began dating independent film producer Stephen Ringer, whom she met on the set of her film Sequoia. The couple became engaged at Big Sur, California, in July 2014. They married in a small ceremony in Portofino, Italy on June 6, 2015. She is a practicing Christian.