Born at Paterson in New Jersey, Reines was educated at the Stevens Institute of Technology and gained his PhD in theoretical physics at New York University in 1944. From 1944 to 1959 he was a group leader at the Los Alamos Scientific Laboratory, concerned with the physics and effects of nuclear explosions.

He was also concerned with investigations into the neutrino, and with Clyde Cowan performed the first experiments that confirmed its existence in the intense radiations from nuclear reactors. The first tentative observation was in 1953, but more definitive experiments were carried out at the Savannah River nuclear reactors in 1956. Detection of the neutrino is difficult because it can travel very long distances through matter before it interacts. Reines subsequently refined the techniques of detection and measurement.

Reines later turned his attention to looking for the relatively small numbers of natural neutrinos originating in cosmic radiation, and to this end constructed underground detectors looking for signs of interactions in huge vats of perchloroethylene. In the course of this work he devised a method of distinguishing cosmic-ray neutrinos from the muons they produce in traveling through the atmosphere.

In 1959 Reines was appointed head of the physics department at the Case Institute of Technology, Cleveland, and was professor of physics and dean of physical sciences at the University of California at Irvine (1966–87).