Shull, who was born in Pittsburgh, Pennsylvania, was educated at the Carnegie Institute of Technology and New York University, where he obtained his PhD in 1941. He began as a research physicist working first for the Texas Company, from 1941 to 1946, and then with the Oak Ridge National Laboratory from 1946 until 1955, when he entered academic life as professor of physics at the Massachusetts Institute of Technology. He remained at the institute until his retirement in 1986.

Shull's main research interest was in the diffraction of slow neutrons by crystals. Just as x-rays can be diffracted by a crystal lattice, neutron beams of suitable energy also show diffraction effects. In the case of x-rays, diffraction is mainly by the electrons in the atom, whereas in neutron diffraction the nuclei scatter the neutrons. From about 1946 onward Shull applied neutron diffraction to determining crystal structure, showing that the method could indicate the position of light atoms such as hydrogen (which are not detected by x-ray methods).

Shull also showed that an additional effect occurred in neutron diffraction – magnetic scattering by interaction of the neutron's magnetic moment with that of the atom. He demonstrated antiferromagnetism in manganese(II) oxide using this technique in 1949. He subsequently did considerable work in ‘magnetic diffraction’ of neutrons and in other aspects of neutron interaction with matter. For his work on neutron diffraction Shull shared the 1994 Nobel Prize for physics with Bertram Brockhouse.