Julius Rosenberg had begun working as an organizer and recruiter of spies and sought help from Greenglass. He convinced David's wife, Ruth Greenglass to visit him in New Mexico and obtain classified secrets about the atomic bomb from her husband, explaining that the information would be passed on to the Soviet Union so that the United States ally would be in a position to better defend itself against Nazi Germany. Ruth returned from her visit with names of scientists involved in the Manhattan Project, locations of test sites and descriptions of different experiments being conducted. She passed this information to the Rosenbergs.

In January 1945, while on leave from New Mexico, Greenglass met with Julius and Ethel. He had been a member of the Communist Party for several years already, persuaded to join by his sister Ethel. Emphasizing the importance of his contributions, Julius took a box of Jell-O and tore it in half marking each half in a particular manner. He gave one half to David Greenglass and told him that a new Soviet contact would be arranged for him, recognizable because the contact would possess the other half of the box.

In June 1945, David was approached by Harry Gold, a Soviet agent who was also gathering information at the time from Klaus Fuchs. Gold showed Greenglass the other half of the Jell-O box as his identification. Greenglass gave Gold the documents that he had procured and Gold, in exchange, gave Greenglass $500.00

In September 1945, Greenglass traveled to New York and met with the Rosenbergs. Here, he gave a detailed description of the Uranium bomb dropped on Hiroshima and the Plutonium bomb dropped on Nagasaki.