In 2000 he passed the second state examination and since 2009 has worked as a lawyer for the Nachmann Vilgertshofer Scharf Barfuß Rechtsanwalts GmbH in Munich.

He chairs the so-called G10 Commission, which takes decisions on the necessity and admissibility of restrictions on the privacy of correspondence, posts and telecommunications pursuant to Article 10 of the Basic Law.

In the negotiations to form a coalition government of the Christian Democrats (CDU together with the Bavarian CSU) and the Free Democratic Party (FDP) following the 2009 federal elections, Mayer was part of the CDU/CSU delegation in the working group on internal and legal affairs, led by Wolfgang Schäuble and Sabine Leutheusser-Schnarrenberger.

[3] In the fourth government under Chancellor Angela Merkel, Mayer served as Parliamentary State Secretary in the Federal Ministry of the Interior under Horst Seehofer from 2018 to 2021.

[5][6] In November 2021, a search committee under the leadership of former President of Germany Christian Wulff recommended Mayer as candidate to succeed Alfons Hörmann as president of the German Olympic Sports Confederation (DOSB); instead Mayer later announced his candidacy as vice-president under the leadership of Thomas Weikert but had to resign shortly after when it was found that the role was incompatible with the German government's cooling-off period for former cabinet members.

Following the 2016 Munich shooting, Mayer called for a review of Germany's gun laws and for stricter enforcement, arguing that "I support stricter regulations on the weapons trade and the creation of a European weapons registry modeled on the German national registry.