Her talent was discovered in 1979 by Swedish musician and TV host Lasse Holm. She was produced by Bert Karlsson's label Mariann Grammofon AB. In 1980, she joined Lasse's group Chips. Chips participated in the Eurovision Song Contest 1982 with the song "Dag efter dag" ("Day After Day"), and reached 8th place.
Andreassen is active in many musical genres such as country, schlager, and musicals. She is mostly famous from Eurovision Song Contest 1985 when she and Hanne Krogh participated as Bobbysocks with the song "La det swinge", and won.
Andreassen has sung in various genres such as country, pop, rock and in musicals. She plays three instruments; guitar, piano and contrabass. She has also been a revue and musical artist.
"Bettan" has a record as the woman with the most participations in the Eurovision Song Contest, tied with Lys Assia, Sue of Peter, Sue & Marc, and Valentina Monetta. She has participated four times, three times in duets (with Kikki Danielsson in 1982, Hanne Krogh in 1985, and Jan Werner Danielsen in 1994) and once on her own (in 1996).