When he was about 11 years old he was placed with Takeda Sokaku, a friend of the monk. This was because Choi showed a great interest in the temple's murals showing battle and martial arts scenes, and Asao was the martial arts teacher of the Japanese royal household and a friend of Kintaro. Takeda adopted Choi and gave him the Japanese name Asao Yoshida, partly to avoid the racial prejudice against Koreans which was rife in Japan.
For 30 years Choi studied with Takeda, most of that time secluded in the mountains, but he also became his father's assistant in teaching high-ranking Japanese. About 1932 he first went overseas, as a member of an exhibition team, to Hawai'i. His father's influence helped Choi avoid the Japanese army draft during World War II, and he was the only member of the school's inner circle, aside from Takeda himself, to survive. After the war Takeda committed suicide in shame for his country's defeat, and advised Choi to return to Korea to live, and this he did shortly afterwards.
He established the modern martial art of Hapkido there, based on techniques learned from Takeda's art of Daito Ryo Aiki Jujutso.