Born in North Adams, Massachusetts, Schwarz was educated at Harvard and the University of California, Berkeley, where he completed his PhD in 1966. After working at Princeton until 1972, he moved to the California Institute of Technology and was appointed professor of theoretical physics there in 1985.

In 1970 Yoichiro Nambu (1921–2015) had proposed that elementary particles may not be particles at all but could be vibrating rotating strings. Schwarz saw in this idea a way to explain the behavior of hadrons – i.e., particles such as protons and neutrons, which respond to the strong nuclear force. Hadrons were known to be composed of quarks and, in the new model, the quarks could be seen as joined by stringlike connections. The theory required space to have 26 dimensions, the existence of a massless particle, and the presence of tachyons (particles supposedly able to travel faster than light). More significantly, however, string theory had to compete with the more plausible account of hadrons proposed by Sheldon Glashow and others, known as quantum chromodynamics (QCD). Against competition from this source string theory withered away.

Schwarz, however, continued to work on string theory and in collaboration with Michael Green reduced the dimensions demanded by early theories to ten. They also eliminated the need for tachyons. As for the massless particle, it seemed to possess precisely the properties demanded by Einstein's theory of general relativity for the carrier of the gravitational force, a fact that was of interest to cosmologists.

Further, the new theory carried other implications. It exhibited a deep symmetry, known as supersymmetry, which seemed able to unify two fundamentally different categories of particles: fermions and bosons. Bosons are not conserved and have an integral spin; fermions are conserved and have a spin of one half. Schwarz demonstrated how they could be seen as waves moving in a closed loop, with fermions moving in one direction and bosons in the other direction.

The fact that the theory needs space–time to have ten dimensions rather than the four observed is explainable if the six ‘extra’ dimensions are extremely small, curled into six-dimensional balls with a diameter of about 10^–35 meter.

Schwarz has published a full account of his work, in collaboration with Green and Ed Witten, in their book Superstring Theory (1987, 2 vols.).