After the death of his father, his mother relocated the family to Munich, where Heinrich began his career as a freier Schriftsteller (free writer).

Mann's essay on Émile Zola and the novel Der Untertan (published over the years 1912–1918) earned him much respect during the Weimar Republic in the left-wing circles, since they demonstrated the author's anti-war and defeatist stance during the World War I, and since the latter satirized Imperial German society; both the novel and the essay became a major impulse for Thomas Mann to write Reflections of a Nonpolitical Man, a work supporting the efforts of the German Empire in the war and condemning Heinrich as one of "Civilisation's Literary Men" (Zivilisationsliteraten), the writers who served the West in its struggle against German "Culture"; later Thomas called the novel an example of "national slander" and "ruthless ruthless aestheticism", while the novel had such admirers as Kurt Tucholsky.

Together with Albert Einstein and other celebrities during 1932, Mann was a signatory to the "Urgent Call for Unity", asking the voters to reject the Nazis.

Einstein and Mann had previously co-authored a letter during 1931 condemning the murder of Croatian scholar Milan Šufflay.

Assisted by Justus Rosenberg, he and his wife Nelly Kröger, his nephew Golo Mann, Alma Mahler-Werfel and Franz Werfel hiked for six hours across the border at Port Bou.

Heinrich styled himself as a socialist revolutionary, Thomas, perhaps precisely because of this, at least in his younger years, gave himself a conservative image.

The two novels describe the life and importance of the highly controversial, yet successful and womanizing king, who is ultimately murdered.

Mann was portrayed by Alec Guinness in the television adaptation of Christopher Hampton's play Tales from Hollywood (1992).