Zinn worked on the Manhattan Project, and is credited with starting the world’s first self-sustaining nuclear reaction by withdrawing a control rod from the world’s first nuclear reactor on December 2, 1942 at the University of Chicago.
After his work on the Manhattan Project, he became the director of the Argonne National Laboratory from 1946-1956. He developed and built several new reactor designs, including the Experimental Breeder Reactor-I - the first nuclear reactor to produce electric power on December 20, 1951.
Zinn received multiple awards for his work, including a special commendation from the U.S. Atomic Energy Commission (1956), the Atoms for Peace Award (1960) and the Enrico Fermi Award in 1969.
The American Nuclear Society (ANS), Operations and Power Division, annually awards their "Walter H. Zinn Award" to recognize the contribution of an individual "for a notable and sustained contribution to the nuclear power industry that has not been widely recognized." Zinn was the first president of the ANS.