AJ was born in Torrance, California. Her father, Mark Michalka, owns a contracting company, and her mother, Carrie Michalka, is a musician, who performed with JC Band (a Christian band). She has German, English, Irish and Scottish ancestry. She is the younger sister of actress and musician Alyson Renae 'Aly' Michalka. Her parents are divorced. As a child, she was raised in Torrance, but briefly lived in Seattle and Milwaukee. AJ and Aly attended Mack Elementary School, although she was homeschooled for a period of time. She also attended a school in Milwaukee, at which she was in a number of musical productions.
AJ has been acting since the age of five, mostly in church play productions, as she was raised as a Christian. Her career began at the age of nine when she landed the leading role in a church musical Jailhouse Rock. AJ has been playing the piano since the age of six, and has played the guitar since her early teens.
In March 2006, AJ made her Disney channel debut in the Disney Channel Original Movie Cow Belles, as character Courtney Callum, along with Aly, who played Taylor Callum. AJ and Aly also appeared in the MTV movie Super Sweet 16: The Movie. AJ has appeared in Oliver Beene, Six Feet Under, The Guardian and General Hospital. She was also featured in The Lovely Bones.
From 2009 to 2013, AJ and Aly performed in a musical duo named 78violet. However, in 2015, they renamed their duo to 'Aly and AJ'. Aly and AJ got cast in Hellcats, starring Ashley Tisdale, until the series was cancelled in 2011.
From 2014-2019, AJ portrayed Lainey Lewis on the ABC series The Goldbergs, and since 2019 stars as the same character in spinoff series Schooled.