Born in Hamburg, Heidemann showed an early interest in photography, and began his career as a freelance photo journalist.

[1] He proved to be a tenacious researcher and a determined traveller to exotic and dangerous locales, but a writer of limited ability.

He sold the rights to Stern for DM 10,000,000 (then approximately US$6 million), along with his tale about how they had been hidden in a barn in East Germany for many years.

[2][7] In 2002, Der Spiegel alleged that Heidemann had worked as a double-agent for the Stasi, and that the publication of the Hitler diaries had been part of a Soviet and East German plan to embarrass and discredit the Capitalist West.

[8] In the BBC Radio 4 programme The Reunion broadcast on 7 September 2008, Heidemann vehemently denied that he had ever been a spy for the Stasi.