Born in Russelsheim, Germany in 1911. He joined the German Communist Party, but fled to England following the rise of the Nazis in 1933. Gaining a doctorate in Physics from the University of Bristol in 1937, he was invited to study at Edinburgh University.
At the outbreak of war, German citizens were interned, Fuchs at a camp in Quebec, Canada. However Professor Max Born of Edinburgh University intervened on Fuchs' behalf. By early 1941, Fuchs had returned to Edinburgh where he was approached by Rudolf Peierls to work on the "Tube Alloys" program, the British atomic bomb research project. He became a British citizen in 1942.
In late 1943 Fuchs transferred to Columbia University, New York City to work on the Manhattan Project. From August 1944 Fuchs worked in the Theoretical Physics Division at Los Alamos, New Mexico under Hans Bethe. His chief area of expertise was the problem of imploding the fissionable core of the bomb. He was present at the Trinity test.
Fuchs passed detailed information on the project to the Russians through Harry Gold in 1945 and further information about the hydrogen bomb in 1947. But it was not until 1948 that it was discovered that the Manhattan Project security had been breached and not until 1949, when Fuchs had returned to England and the Harwell Atomic Energy Research Establishment, that he was confronted by intelligence officers as a result of the cracking of Soviet ciphers known as theVENONA project. Fuchs was prosecuted by Sir Hartley Shawcross. Fuchs confessed in January 1950 and was convicted on March 1, 1950 and sentenced the next day to fourteen years in prison, the maximum possible for passing military secrets to a friendly nation.