She was born in Turinge, about 40 km southwest of Stockholm, and moved around Sweden for much of her youth. Liljeberg's mother was relatively young when she was born, and her parents divorced when she was one year old. At the age of 9 (1991), Liljeberg began her acting career when she won a role in the series "Sunes jul". Between 1993 and 1997, she became played in amateur theatre, which she continued with until gaining a role in the movie "Närkontakt" later in 1997. The following year she won her breakthrough role in the Lukas Moodysson movie "Fucking Åmål". For the movie she stopped his study in high school to take up the role, which ultimately won her the 1999 Guldbagge Award for Best Actress, together with Alexandra Dahlström. After her roles in "Fucking Åmål" and "Sherdil", Liljeberg began an adult education course in order to graduate from high school. While studying, she also played in "Bear's Kiss", and voiced a character in the Swedish version of IMAX movie "T-Rex: Back to the Cretaceous". In 2002, she completed the adult education course, and is now studying medicine at the Karolinska Institute to become a pediatrician.