Allison Joy Langer was born in Columbus, Ohio. At age 5, her family moved to the San Fernando Valley, just outside of Los Angeles, where she attended Granada Hills High School. She first began using her initials to join an all-boys baseball team, and the nickname stuck. She was introduced to Ernie Lively, who became her acting coach.

After roles on television shows, including Drexell's Class (1991) (which starred a then-unknown Brittany Murphy), Coach (1989), and The Wonder Years (1988), she gained international fame on the series My So-Called Life (1994) as Rayanne Graff. Though a critical hit, the series was canceled after only 19 episodes, and Langer moved on to several more short-run series. She married Charles Peregrine Courtenay, a practicing attorney and British peer, son of the 18th Earl of Devon. When her husband became the 19th Earl of Devon, she became the Countess of Devon. She had taken several years off to raise the couple's two children, but, in 2011, she returned to acting and joined the cast of Private Practice (2007).

In January 2014, she permanently relocated to London with her husband and children.