During her almost 23 years as a member of the European Parliament Thyssen was re-elected five times. She has sat on the European Parliament's Committee on the Internal Market and Consumer Protection, has substituted for the Committee on International Trade, was a member of the Delegation to the EU-Ukraine Parliamentary Cooperation Committee and a substitute for the Delegation to the EU-Turkey Joint Parliamentary Committee. From 1999-2014 she served as leader of the European People's Party's (EPP) Belgian delegation and from 2004-2009 was elected first vice-president of the EPP.