He went on to serve in a variety of roles, firstly as Commission Spokesperson for Information Society and Media, before becoming head of cabinet to the Commissioner for Justice, Fundamental Rights and Citizenship, Viviane Reding.

He was appointed by the commission, as a principal adviser to the Directorate-General for Economic and Financial Affairs, and as a director at the European Bank for Reconstruction and Development.

[7] Shortly after his appointment, various media reports began to profile Selmayr as a highly influential figure within EU politics, with Politico describing him as "the most powerful EU chief of staff ever" in November 2016, noting that even Jean-Claude Juncker jokingly referred to Selmayr by the nickname "the Monster.

Minutes after this appointment, Juncker informed the European Commissioners that the then secretary-general, Alexander Italianer, intended to retire; they had not been previously notified of this.

[12] On 1 March 2018, following the formal retirement of Italianer, Selmayr was approved by the College of Commissioners to replace him as secretary-general.

[14] The president of the European Commission Jean-Claude Juncker reportedly threatened to resign if the controversy led to Selmayr's dismissal.

[18] In the immediate aftermath of his appointment as secretary-general of the European Commission, it was reported that Selmayr had been editing Wikipedia using his own name from an account using a verified ec.europa.eu email address.