Taketani Mitsuo graduated from Kyoto University in 1934 with a degree in physics. He pursued research in nuclear and elementary particle theory as a collaborator with Hideki Yukawa and Shoichi Sakata. Meanwhile, along with Shoichi Nakai and Osamu Kuno, he was arrested twice for participating in the anti-fascist magazines Sekai Bunka and Saturday. During the Second World War, he initially held a position at his alma mater, then moved to RIKEN, where he became assistant to Yoshio Nishina.