Within the German Bundestag, Petry is spokesman on European affairs for the SPD parliamentary group.

From 1992 to 1995 Petry worked as a research assistant for the SPD parliamentary group in the State Parliament of Saarland under Reinhard Klimmt, and from 1995 to 1998 he served as the personal assistant and press spokesman of the Saarland's State Minister of the Interior, Friedel Läpple, after which he worked as press spokesman for the SPD parliamentary group in the State Parliament until 1999.

He was subsequently a senior officer in the field of citizenship and civil status matters and foundation law in Saarland's State Ministry of the Interior.

In addition to his committee assignments, Petry is a member of the German-French Working Group on the Élysée Treaty[6] and the Inter-Parliamentary Conference on Stability, Economic Coordination and Governance in the EU.

In 2020, Petry also joined the German delegation to the Parliamentary Assembly of the Council of Europe (PACE).