Michalka was born and raised in Torrance, California and is the younger sister of actress and musician, Aly Michalka. Her father, Mark, owns a contracting company, and her mother, Carrie, is a musician and performed with JC Band, a Christian band. Her parents are divorced. As a child, she briefly lived in Seattle and Milwaukee. Both she and her sister attended Mack Elementary. She has been playing the piano since she was six and started playing the guitar in her early teens. She started acting when she was five years old, mostly in church play productions. She was raised as a Christian and was homeschooled for part of her childhood. Michalka was in a number of musical productions at her school in Milwaukee as a young adult.
She plays a variety of instruments such as acoustic guitar, electric guitar, piano, and bongos. She was a model for catalogs. In March 2006, she made her Disney Channel debut in the Disney Channel Original Movie Cow Belles, as "Courtney Callum," along with sister Aly, as "Taylor Callum." She has also made appearances on shows such as Oliver Beene, Six Feet Under, The Guardian, and General Hospital. She has also appeared with her sister, Aly, in the MTV film, Super Sweet 16: The Movie.
In 2009, she was featured in the Peter Jackson film The Lovely Bones, released widely on January 15, 2010.
From 2009 until 2013, Michalka and her sister performed as the musical duo 78violet. They decided to later change their name back to "Aly and AJ" in 2015. She was cast as a recurring character on The CW series Hellcats starring Ashley Tisdale and along with her sister Alyson Michalka. The series ran for one season and was cancelled in May 2011.
Michalka appeared in the 2011 film Super 8, directed by J. J. Abrams. Her first big-screen starring role is as the title character in 2013's Grace Unplugged.
Since 2013, Michalka has had a recurring role on the ABC sitcom, The Goldbergs, as Lainey Lewis, the girlfriend of one of the main characters on the show. In May 2015, it was announced that she would be a series regular for the show's upcoming third season. In July 2014, she and her sister, Aly Michalka, filmed an untitled drama in Laurel Canyon, California. The film was directed and written by Aly's husband, Stephen Ringer. Michalka was one of the producers of the film, alongside her sister and Ringer, and also participated in the casting process. In early May 2015, it was announced the film, now titled, Weepah Way for Now, premiered at the Los Angeles Film Festival on June 16, 2015. The film was later released on DVD, iTunes, and other streaming platforms in June 2016.
Michalka also voices Stevonnie, the fusion between titular character, Steven, and his friend, Connie, on the popular television series Steven Universe on Cartoon Network.
Michalka resides in Los Angeles, CA and is a practicing Christian.