In 1991, Thyssen became a Member of the European Parliament (MEP) for Flanders with the CD&V, the Flemish Christian Democrats in Belgium; she took the place of , who had moved to the Belgian Senate. From 1995-2008 she served as municipal Councillor for Oud-Heverlee, a municipality located in the Belgian province of Flemish Brabant and in 2001 she became First Alderman for Oud-Heverlee. As a local councillor she chose to work aon such social issues as childcare and care for the elderly. In 2008 Thyssen was elected leader of CD&V party.