Rainer started his career as Trainee with Deutsche Bank AG in Hanover. Between 1983–87, he worked as assistant at the Institute of international procedure and arbitration in Munich. Then he served as an Education Editor at Deutsche Journalistenschule (GermanJournalistsSchool) in Munich. In the year, 1987–89, Rainer served as Lawyer in the international law offices Nörr, Stiefenhofer & Lutz in Munich and Stegemann, Sieveking & Lutteroth in Hamburg. He then completed his PhD in International Law at t he University of Regensburg in 1989. 

Rainer then joined as Editor in Chief of Business Law Europe and Tax Letter Europe, two legal news services being published in five languages at Bertelsmann International in Munich in the year 1989.

He became General Manager of Spotlight Publishing House and Publisher of the Magazines Spotlight, écoute, ECOS and adesso between 1992-1995 and served as General Manager for Main Post Publishing House until 1999.

Rainer then joined DIE ZEIT as CEO since 1999.