At this time, he was a member of the "Gréng Alternativ Partei" (green party) and was active in the student organisation UNEL, where he was part of the national buro.

In 1999 he joined Jacques Santer's cabinet in the European Parliament and in 2001 he became the Secretary General of the CSV parliamentary group.

He has written three books on European issues: the 'Europa in der großen Krise Ein Drama in drei Akten' ('Europe in the Great Crisis - A Drama in Three Acts') in 2009,[4] 'Die Schulden überwinden - Ein Haushalt für den Euro' ('Overcome the debt - A budget for the euro') in 2012 [5] and 'Projet de Manifeste pour les États-Unis d'Europe' ('Manifesto for a United States of Europe') in 2013.

[6] He has also contributed to the book "Europe's Next Avoidable War: Nagorno-Karabakh" with the chapter "The Karabakh Dilemma: Right to Self-Determination, Imperative of Territorial Integrity, or a Caucasian New Deal?".

[7] In February 2018, Engel went to Nagorno-Karabakh as part of a so-called friendship group with three other MEPs to observe a referendum vote on changes to the constitution.