After a postdoc with Charles Townes at Columbia Radiation Laboratory, he pursued the tenure track at Johns Hopkins University.
In 1971 he returned to New York, where he hold from 1974 to 2004 a Distinguished Professorship at the City College.
Starting in 1963, Cummins was among the first to use laser light scattering for the systematic investigation of condensed matter.
In the 1980s he started to study growth processes and the dynamics of viscous liquids.
In close collaboration with theorist Wolfgang Götze, he provided experimental support for the mode-coupling theory of structural relaxation.