Reimertz is the grandnephew of Nikolaus Groß, resistance fighter in the 20th July plot against Hitler. His grandfather was a democratic major and politician from Westphalia. His father was a mining engineer and met his mother at RWTH Aachen; she was a pharmacist from Riga and of Baltic German ethnicity. Reimertz was baptized by auxiliary–bishop Friedrich in front or the Throne of Charlemagne at Aachen Cathedral and raised at Kronberg, the widow seat of the German empress, where he received an intense artistic and musical training. Stephan was a boy scout in the German Scout Association Saint George, a competition swimmer and amateur boxer, an experience he described in his autobiographical novel Papiergewicht.
At junior high, he co-founded a student magazine to prevent the downgrading of his secondary school into a comprehensive school and to go up against the so-called Change Through Rapprochement policy of the West-German government. Reimertz was a committed opponent of the Second Vatican Council, of the Movement of '68 as well as of the americanisation of Germany. During the West German federal election, 1980 he supported Franz Josef Strauß, candidate for chancellor of the Christian Social Union of Bavaria. His philosophical teachers where Wolfgang Stegmüller, Robert Spaemann, Eugen Biser and Rudolf Schottlaender, whom he visited in East-Berlin regularly. With his tutor in the classics, Uvo Hölscher, Reimertz shared a lifelong passion for the work of Friedrich Hölderlin.
A grantee of the German National Merit Foundation, the German Academic Exchange Service, etc., Reimertz went to college at LMU Munich and graduated with an MA in comparative literature and a doctorate in art history and philosophy from FU Berlin. He has taught at Juniata College in Huntingdon, Pennsylvania, and was a Fulbright grantee at University of Texas at Austin, and a research fellow at Washington University in St. Louis, Missouri.
During the German Reunification of 1989, Reimertz was a host and speaker in major broadcasts and contributed to national dailies.
Reimertz has written widely read novels and biographies as well as a cultural history of tea. In 2001, he authored Papiergewicht (“Paper Weight”), a novel on the social changes of the early Seventies, set in a German upper-class family. The novel features the 1971 New York boxing fight between Americans Muhammad Ali and Joe Frazier, the so-called Fight of the Century.
In 2003 he published Max Beckmann: A Biography, the first about the German artist.
- Eine Liebe im Porträt. Rowohlt, Reinbek 1996. ISBN 3-499-22768-1
- Papiergewicht. Luchterhand, Munich 2001, ISBN 3-630-87086-4
- Die Frauenfalle. Hey Publishing, Munich 2013, ISBN 978-3-942822-41-1
- Max Beckmann, (rororo-Monographie). Rowohlt, Reinbek 1995. ISBN 3-499-50558-4
- Vom Genuß des Tees. Eine heitere Reise durch alte Landschaften, ehrwürdige Traditionen und moderne Verhältnisse, inklusive einer kleinen Teeschule. Gustav Kiepenheuer, Leipzig 1998, ISBN 3-378-01023-1
- Woody Allen. Eine Biographie. Rowohlt, Reinbek 2000, ISBN 3-499-61145-7
- Max Beckmann. Biographie. Luchterhand, Munich 2003. ISBN 3-630-88006-1
- Woody Allen (rororo-Monographie). Rowohlt, Reinbek 2005, ISBN 3-499-50410-3