After studying at the Leipzig University of Music and Theatre (1990–1994), Westermann debuted at Hamburg's Ernst Deutsch Theater, performing in "Der Freigeist" (directed by Franziska Kohlund) and "Andorra" (Valerij Grishko). She also appeared at Chemnitz City Theater and Gera's municipal stages.

From the mid-1990s, she took on growing TV and film roles. She impressed as a humiliated daughter in Bernd Schadewald's gripping courtroom drama "Angst" (1993), about a man killed by his family on prison release, alongside Christian Redl and Jürgen Vogel. Schadewald also cast her in his psychothriller "Kinder des Satans" (1995). After a lead in "Gesches Gift," she starred with her husband Aljinovic in the modern fairy tale "Der Strand von Trouville" (1997), joined the ensemble of Heinrich Breloer's docudrama "Todesspiel," and played a small role opposite Martina Gedeck in the crime film "Lea Katz – Das wilde Kind" (1998).

Westermann soon landed series roles in "Sylvia – Eine Klasse für sich," "Zwei Brüder," "SK Babies" (all 1998), and "Mordkommission" (1999). She then starred in the acclaimed adventure drama "Gran Paradiso – Das Abenteuer Mensch zu sein," shot at 4,000 meters altitude under Miguel Alexandre. Above-average TV work followed with Hans-Christoph Blumenberg's docudrama "Deutschlandspiel" (2000) and the "Bella Block" episode "Am Ende der Lüge" (also 2000). Another crime film, "Tödliches Vertrauen," came in 2002. In 2003, she delivered a standout performance as an overwhelmed young mother in the youth film "Wer küsst schon einen Leguan?" – a role that earned her an Adolf Grimme nomination for Best Actress.