Born in Nagasaki in 1943, Tomonaga graduated from Nagasaki University School of Medicine in 1968. In 1968, he joined the Department of Hematology (JAERI) at the Nagasaki University Research Institute for Post-Atomic-Bomb Disease Medicine (now Nagasaki University Post-A-bomb Disease Medical Research Institute). Since then, he has been engaged in research on A-bomb survivors and leukemia as a hematologist for more than 40 years. After serving as a lecturer and assistant professor in the Department of Internal Medicine at JAERI, Tomonaga became a professor in 1990, and in 2009 became the director of the Japanese Red Cross Nagasaki Atomic Bomb Survivors Hospital. He is a member of International Physicians for the Prevention of Nuclear War (IPPNW) and is actively involved in the campaign for nuclear abolition. 2 years old, he and his mother were exposed to the atomic bomb at 2.7 km from the hypocenter. Tomonaga has been the director of the Junshin Seibokai Nagasaki Atomic Bomb Home Clinic since 2002. He is also the Japanese government representative to the International Conference on the Inhumanity of Nuclear Weapons and the International Vice President of International Physicians for the Prevention of Nuclear War (IPPNW). He is currently a member of the Ministry of Foreign Affairs' “Wise Men's Council to Promote Substantive Nuclear Disarmament. He is also a representative of the Global Citizens Assembly for Nuclear Phase-Out Nagasaki and a visiting professor at the Research Center for Nuclear Weapons Abolition (RECNA), Nagasaki University.