He was a member of the team that built Chicago Pile-1, the world's first artificial nuclear reactor, and was one of those present on December 2, 1942, when it achieved criticality.

After the war he received his doctorate, and became a researcher at the Argonne National Laboratory from 1947 to 1950, at Massachusetts Institute of Technology from 1951 to 1958, and at University of Illinois at Urbana–Champaign from 1958 to 1986, where he pursued studies related to the atomic nucleus.

A politically active student, he organized strikes and a boycott of his own 1938 graduation ceremony in protest against the City College president's Italian Fascist sympathies.

Enrico Fermi asked him to join the group at Columbia working on the nuclear fission of uranium that also included Herbert L. Anderson, Bernard T. Feld, Leo Szilard and Walter Zinn.

He assisted in the construction of Chicago Pile-1, the world's first artificial nuclear reactor, and was one of those present on December 2, 1942, when it achieved criticality.

In July 1945, he was one of the signatories of the Szilard petition,[1] which urged that "the United States shall not, in the present phase of the war, resort to the use of atomic bombs.

[1] With the war over, Wattenberg returned to his studies,[2] completing his PhD at the University of Chicago under the supervision of Walter Zinn.

[13] Rather than work in academia, he chose to join Fermi at the Argonne National Laboratory, where he helped design and build nuclear reactors.

[14] By 1950, the rise of McCarthyism led to Wattenberg leaving Argonne, first to go to the University of Illinois at Urbana–Champaign for a year, and then to MIT, where he remained until 1958.

He used MIT's synchrotron to study the properties of nucleons and K-mesons, gaining important insights that would later be incorporated into the standard model.

He made frequent appearances on Studs Terkel's radio show, and in NPR's All Things Considered, usually on the occasion of the anniversary of Chicago Pile-1 going critical, or of the atomic bombings of Hiroshima and Nagasaki.