Was sent by Britain to the United States to work further on the atomic research project in late 1943 at Columbia University. Met with a new Soviet contact in the United States named Harry Gold (known to Fuchs as Raymond). Delivered research notes to Gold each month, while in New York and later after being transferred to the Los Alamos laboratories in New Mexico where actual development of the bomb took place. Stepped up the meetings with Gold, delivering information each week to him.

Delivered technical schematics of the atomic bomb as well as reports on test blasts. Also delivered information about an experimental plutonium bomb. Ended his meetings with Gold after the United States dropped atomic bombs on Hiroshima and Nagasaki.

After returning to England in 1946, served as the head of the theoretical physics division of the Atomic Energy Establishment in Harwell. England was in the process of developing its own atomic weapon and wanted Fuchs contributing to the effort. Continued to supply the Soviet embassy with documents and information, keeping them abreast of British innovations, despite the fact that World War II had ended.

Was assigned a new contact in Britain, code-named "Sonia." Sonia was in fact Ruth Kuczynski, a very successful Soviet spy throughout Europe. Sonia proved to be a difficult taskmaster, more and more demanding of Fuchs for information. Fuchs became disenchanted with Soviet requests, believing that their use of the information was nowhere nears as noble as he had envisioned. Eventually severed contact with Sonia any Soviet agents.