Born and raised in Hiroshima Prefecture, Tsuboi was interested since childhood in mathematics and science and wished to be an inventor.

However, the only assistance he could find there was policemen pouring cooking oil onto the skin of school children to ease the pain from burns.

When her uncle suggested leaving and holding a funeral for him, his mother began running around screaming his name.

[10] After retirement from teaching, Sunao Tsuboi became more involved in anti-nuclear and anti-war activism, participating in sit-ins, demonstrations and rallies.

He travelled across Japan and abroad in his many campaigns and repeatedly stated that he wanted nuclear weapons to be abolished.

[14][9] In recognition for his work campaigning for the abolition of nuclear weapons and support for other hibakusha, he was named an honorary resident of Hiroshima in 2018.

[12] He had criticised Japanese Prime Minister Shinzō Abe for attacking Japan's postwar commitment to pacifism.

[15][16] When he was 26 years old and teaching at a college for women, Tsuboi met his future wife, Suzuko.

[1][7] Her parents objected to the marriage as they were concerned that, as an atomic bomb survivor, he would soon die and leave her a widow.