In May 2007 media scholar Stefan Weber accused Hahn of copying numerous pages of his PhD thesis from other books without citing them correctly.

[8] The University of Vienna looked into the matter but decided not to start an examination procedure, saying that Hahn never presented "other people's intellectual property" as his own.

[9] Renewed allegation of plagiarism emerged in February 2011, when Austrian Green MP Peter Pilz commissioned Stefan Weber to re-assess Hahn's PhD thesis.

In the same years (1997–2003) he served as a member of the supervisory board at Novomatic, an online gaming and casino management system firm, of which he was appointed CEO in 2003.

[31] One of Hahn's aims has been to raise public awareness of the great number of successful women in science and research in Austria by means of the so-called "Visibility Measures" which were launched in 2008.

[40][41][42][43] In 2008 Hahn created the "Award of Excellence" for the best doctoral theses in Austria, as a way of recognising the work of young scientists and making the public more aware of their activities.

[49] Hahn himself stated that joining ESO had been one of the most significant political decisions in the field of research for 20 years, from which not only astronomers but also physicists, mathematicians, software specialists and high-tech companies would benefit.

[66] In November 2009 Hahn was nominated by the Austrian Federal Government to succeed Benita Ferrero-Waldner as member of the European Commission from Austria.

Reportedly, while the ÖVP's first choice for Commissioner would have been Wilhelm Molterer, this was vetoed by chancellor Werner Faymann, opening the road for Hahn.

Based on the new regulation, European regions have developed so called smart specialisation strategies to identify their particular strengths and potentials as the basis for investments.

Despite cuts in the overall EU budget, Hahn succeeded to keep the high level of financial support for strategic investments in the new period (2014–2020): Funding for regional and cohesion policy in 2014–2020 amounts to 351,8 billion Euro.

Taking into account the national contribution of member states, and the leverage effect of financial instruments, the overall impact of investments is likely to be about 600 billion Euro.

Thanks to increased flexibility, allowing reprogramming of structural funds, the reformed Cohesion Policy paid also an important role in overcoming coping with the economic crisis.

[72] [73] Despite cuts in the overall EU budget, Hahn succeeded to keep the high level of financial support for strategic investments in the new period (2014–2020): Funding for regional and cohesion policy in 2014–2020 amounts to 351,8 bn EUR.

In his new portfolio, Commissioner Hahn launched,[76] together with High Representative Federica Mogherini, a comprehensive reform of the European Neighbourhood Policy (ENP), as mandated by Commission President Jean-Claude Juncker.

[79] Shortly after, Austria's interim Chancellor Brigitte Bierlein announced her government's intention to put forward Hahn for a third term as the country's representative in the European Commission.