During his tenure as AEC Chairman, Dean defended J. Robert Oppenheimer at Los Alamos against McCarthy-era accusations. As Cold War tensions heightened and the Korean War raged on, Dean led a massive industrialization of the United States nuclear facilities. The hydrogen bomb, a nuclear weapon of massive and unprecedented force, was perfected during his tenure.

On leaving government service, Dean joined Lehman Brothers and later General Dynamics. He was killed in a commercial aviation accident on August 15, 1958.