Tomonaga was born in Tokyo on 31 March 1906 into a wealthy family. From an early age, he received a great cultural education from his father, Sanjuro Tomonaga, a philosopher and teacher.
After completing his secondary education at the High School in his hometown, Tomonaga entered Kyoto Imperial University in 1926, graduating with a degree in physical science in 1929. There, he met Hediki Yukawa, who was to win the Nobel Prize in Physics in 1949 for formulating the meson hypothesis, based on theoretical work on nuclear forces.
After graduating, he continued his research at Kyoto Imperial University for three years, and in 1931 he moved to the Research Institute of Physics and Chemistry in Tokyo, where he worked under the supervision of Dr. Yoshio Nishina, who introduced him to the world of quantum physics.
In 1937, he began working at the University of Leipzig, collaborating with Werner Heisenberg's research group. Two years later, due to the outbreak of World War II, he returned to Japan, where he completed his doctoral thesis on the study of nuclear materials at the University of Tokyo.
From 1940 onwards, he began an in-depth study of mesons (particles of intermediate mass between the electron and the neutron), which allowed him, between 1942 and 1943, to introduce a series of fundamental corrections in the theory of quantum electrodynamics. While carrying out this work, he began a brilliant teaching career at the University of Bunrika (Tokyo), later absorbed by the University of Tokyo, where Tomonaga continued to teach physics.
In 1948, together with his students, he re-examined a paper by Sidney Dancoff on quantum electrodynamics and discovered the renormalization method, independently obtaining the same results as Julian Schwinger. The following year Tomonaga was invited by Robert Oppenheimer to work at the Institute for Advanced Study in Princeton Town (New Jersey), where he led a team devoted to the study of another quantum particle, the fermion.
In 1955, back in Japan, Tomonaga became Director of the Nuclear Institute of the University of Tokyo, and a year later he was appointed Rector of the University of Tokyo, a position he held until 1962. Shortly afterward, he was appointed President of the Science Council of Japan by his nation's political authorities, an institution from which he issued numerous proclamations in defense of the peaceful use of nuclear energy. He was also director of the Institute of Optics at the University of Tokyo.
In 1965, together with Julian Seymour Schwinger and Richard Phillips Feynman, he received the Nobel Prize in Physics for their fundamental work in the study of quantum electrodynamics, which led to an in-depth study of the physics of elementary particles.
In addition, Tomonaga received the Prize of the Japanese Academy of Sciences, the Order of Culture of the Japanese Government, and the Lomonosov Medal, awarded by the political and cultural authorities of the Soviet Union. A member of the Japanese Academy of Sciences, he was also a member of such prestigious international scientific societies as the Leopoldina Academy (Germany), the Royal Swedish Academy of Science, the Bayerische Akademie der Wissenschaften (Germany), and the National Academy of Science of the United States of America.
He published several articles and treatises disseminating his discoveries, most of them related to quantum dynamics. His major works include Photoelectric Production of Positive and Negative Electrons (1934), Relativistic Invariant Formation of Quantum Wavefield Theory (1946), and, above all, Quantum Mechanics (1962).
Tomonaga died of cancer on 8 July 1979 in Tokyo at the age of 72.