He took up photography in 1934 at the urging of his brother, de:Walter Gernsheim, who thought it a more practical profession for someone from a partially Jewish background who intended to leave Nazi Germany.

At the outset of the Second World War, Gernsheim was deported to Australia on the HMT Dunera and interned as a "friendly enemy alien" for a year at Hay in New South Wales,[5] along with other German nationals including the artist Ludwig Hirschfeld Mack of the Bauhaus, Heinz Henghes (sculptor), Hein Heckroth (film and stage designer), George Teltscher (graphic artist), Klaus Friedeberger (painter), tenor Erich Liffmann, the composer Ray Martin, the artist Johannes Koelz, the photographers Henry Talbot and Hans Axel, the art historians Franz Phillipp and Ernst Kitzinger, the author Ulrich Boschwitz, the furniture designers Fred Lowen and Ernst Roedeck, and Erwin Fabian (sculptor).

Gernsheim earned his release from internment by volunteering to work for the National Buildings Record, returning to London in 1942 to photograph important monuments with a view to revealing their artistic merits.

In 1945, at Beaumont Newhall's prompting, Helmut and Alison Gernsheim started collecting the works of historic photographers, especially British ones, which were disappearing.

They amassed a huge collection containing work by Julia Margaret Cameron, Alvin Langdon Coburn, Hill & Adamson, William Henry Fox Talbot and Louis Daguerre.

In the end, after many fruitless discussions with authorities and potential sponsors in several countries, he sold everything to the University of Texas at Austin in 1963 where it formed the basis of a new Department of Photography at the Humanities Research Center.