Born in New York, Weinberg was educated (as was Sheldon Glashow) at the Bronx High School of Science, at Cornell, and at Princeton, where he gained his PhD in 1957. Following appointments at Columbia (1957–59), Berkeley (1959–69), the Massachusetts Institute of Technology (1969–73), and Harvard (1973–83), he was appointed professor of physics at the University of Texas, Austin.

In 1967 Weinberg published a paper, A Model of Leptons, which proposed a unification of the weak and electromagnetic interactions since known as the ‘electroweak theory’. In modern particle physics forces operate through the interchange of particles: the electromagnetic force by interchanging photons, and the weak force by the interchange of the W and Z bosons. The claim that the forces had been united into a single force would imply that photons and bosons belonged to the same family of particles. But it is only too clear that this could not be the case; the photon was virtually massless, while the bosons were even more massive than the proton.

The difference was explained by Weinberg in terms of spontaneous symmetry breaking (SSB). At the extremely high temperatures present shortly after the big bang photons and bosons would have been indistinguishable. At some point during the cooling the initial symmetry was spontaneously broken, and during this breakage some particles acquire different properties. Weinberg likens the process to what happens when a piece of iron is cooled below a temperature of 770°C. Below this point the material becomes ferromagnetic and a magnetic field pointing in some unpredictable direction can appear, spontaneously breaking the symmetry between different directions.

The question of the origin of the mass of the bosons remained. Weinberg proposed that the Higgs mechanism, described by Peter Higgs in 1964, though hypothetical, would suffice. As a consequence of Weinberg's theory the existence of ‘neutral weak currents’ was predicted. It has previously been supposed that weak interactions invariably involved a transfer of electric charge carried by the bosons W⁺ and W^–. In electromagnetic interactions the photon is exchanged setting up a neutral current. The weak interaction should be able to proceed in the same way with the transfer of the neutral boson Z⁰. Neutral weak currents were first observed in 1973, and the bosons of the electroweak theory were detected by Carlo Rubbia at the European Laboratory for Particle Physics in 1983. It was for this work that Weinberg shared the 1979 Nobel Prize for physics with his schoolmate from the Bronx, Sheldon Glashow, and with Abdus Salam.

Weinberg has also worked in the field of cosmology, publishing in 1972 a substantial treatise on the subject, Gravitation and Cosmology. This was followed by The First Three Minutes (1977), an extremely popular account of the three minutes following “about one hundredth of a second after the beginning when the temperature had cooled to a mere hundred thousand million degrees Kelvin.”

In a later work, Dreams of a Final Theory (1993), Weinberg argued that in today's theories we are already beginning to catch glimpses of the outlines of a final theory. Whether or not the glimpses are of shadows or something more substantial, Weinberg suggests, depends upon whether the US Government goes ahead and constructs the 8-billion-dollar Superconducting Super Collider in Ellis County, Texas. When complete and running the SSC should be sufficiently powerful to reveal the Higgs boson.