Mack was born in Germany to American parents, Mindy and Jonathan Mack. Her father was an opera singer performing there. The Mack family moved back to the United States when she was two years old.

Her first major TV role came in an episode of the WB series 7th Heaven, in which she gained a lot of attention playing a teenager who cut herself. In 2000, she starred in two short-lived series, Hiller and Diller and Opposite Sex. She also starred beside Sam Jones III in an R. L. Stine miniseries The Nightmare Room. Her filmography includes Eric Stoltz's directorial debut My Horrible Year!, in which she plays a girl having great difficulties in her life as she turns sixteen, and Camp Nowhere.

Mack stars as Chloe Sullivan, Clark Kent's best friend, in the CW hit series "Smallville". In the summer of 2006, Mack's voice was heard as the sister of the main character in the Warner Bros. CGI movie The Ant Bully. That year she also provided the voice of a museum curator named Clea in an episode of The Batman (who bore a deliberate resemblance to Mack).

Mack and co-star Kristin Kreuk have become close friends. Allison is also an accomplished dancer and singer, and currently lives in Vancouver. She enjoys reading, going to movies and plays and spending time with her friends and family. Allison's engagement to her long-time boyfriend Peter (who is in the band "Leisure") was announced in 2003, but the current status of their relationship is unconfirmed.