The photo lab and darkroom were founded by Leo Cohn, who had worked as a professional photographer in Germany and managed a shirt factory. No more details are available on his life and work before his emigration and all that is known is that he emigrated to New York around 1939 and “went into business as a photofinisher, with Black Star picture agency as his first customer. His first laboratory was the kitchenette of his apartment on the 94th Street in New York City. From the first, his policy was to offer a high type of photofinishing service at a reasonable price to professional photographers. There was a real need for the service” (Kelley 1947, 192). From October 1940, he ran his business from the Salmon Tower Building at 11 West 42nd Street. It was opposite the New York Public Library and near Central Station, close to such photo agencies as Black Star, Rapho Guillumette, PIX, Three Lions and Graphic House and also close to the editorial headquarters of magazines like U.S. Camera, Life and Popular Photography.

As not every émigré photographer developed and enlarged his images by himself in a studio and there was a need for images during the 1940s, Leco Photo Service provided an important service and also acted as a contact hub. While they waited for their pictures to be processed, the photographers had the opportunity meet and get to know other photographers, editors and writers. Among the lab's émigré clients were David Seymour (Chim), Herbert Gehr, Philippe Halsman, Fritz Henle, Ylla, Eric Schaal, Elli Marcus and Henry Rox as well as Andreas Feininger.
After his arrival in New York in 1939 David Seymour worked at Leco where he enriched his knowledge of photo chemistry and physics in fine-grain developing as well as learning new, faster methods of print control (Bing 1996; Beck 2005).

Besides David Seymour, the German émigré Erika Stone also started her later career as a photographer working as a darkroom assistant at Leco. Articles in Popular Photography on Leco’s photo lab, and on work opportunities for women in the photo sector and women photographers, cites Leco Photo Service as a business where, in 1947, the staff were mainly women. Among them were Ilse Heller, Ilse Nothman, Ilse Rumpler, Ilse Steinitz, Bernd Steinitz, Jeanne Granolles, Marcelle Alexander and Seymour Levine. (Kelley 1945; Kelley 1947) One article reveals that many women and emigrants were hired by photographic businesses. We know from other émigré women, such as Ruth Bernhard and Erika Stone, that photofinishing and darkroom work were largely female occupations and provided a starting point for émigré and non-émigré women in the photo business and a subsequent career as a photographer. So, it is possible that other women employed as assistants at Leco also went on to careers as photographers, though no names or details are available.