Berès was a Member of the European Parliament since from June 1994 to July 2019. She was first elected as the second listed candidate of the Socialist Party in the 1994 elections and was re-elected in 1999, 2004, 2009 and in 2014 when she was the head of her party's list in the Île-de-France constituency. She was head of the French socialist delegation (1997-June 2004 and 2014-2017) and Vice-President of the Socialist group in the European Parliament (1997-2004). She chaired the European Parliament's Committee on Economic and Monetary Affairs from 2004 to 2009 and the Committee on Employment and Social Affairs from 2009 to 2014. From December 1999 to October 2000, she was the Vice-President of the European Parliament Delegation to the Convention in charge of elaborating a European Union Charter of Fundamental Rights. She was also a member of the European Convention in charge of drafting a Constitution for Europe from February 2002 to July 2003.
A Member of the diplomatic team of the presidential campaign of François Hollande, she opposed of the renegotiation of the European Fiscal Pact, an important issue for Hollande, believing that "we do not renegotiate the signature of France, especially in Europe where we are a founding country."