Jay Wechsler enlisted in the Army in 1943 and spent several months at Oak Ridge working as an Army construction engineer, In the winter of 1943, he was suddenly transferred to Los Alamos, where he began working directly with Otto Frisch. Wechsler helped Frisch work on a large fission chamber that Frisch had originally designed in Denmark and later shipped to the United States. He was also an explosives expert at Beta Site, testing what would later become the implosion system for the plutonium bomb.
Wechsler continued his weapons work for the government throughout the Cold War.