Tomonaga, who was born at Tokyo in Japan, graduated from Kyoto University in 1929 and then went to work in his native city. He remained there for the rest of his academic career, becoming professor of physics in 1941 and president of the university in 1956.

He was one of the first to develop a consistent theory of relativistic quantum electrodynamics. The problem at the time was that there was no quantum theory applicable to subatomic particles with very high energies. Tomonaga's first step in forming such a theory was his analysis of intermediate coupling – the idea that interactions between two particles take place through the exchange of a third (virtual particle), like one ship affecting another by firing a cannonball. Between 1941 and 1943 he used this concept to develop a quantum field theory that was consistent with the theory of special relativity. However, World War II prevented news of his work from reaching the West until 1947, at about the time that Richard Feynman and Julian Schwinger published their own independent solutions to the same problem. All three shared the Nobel Prize for physics for this work in 1965.