Kaicho Nakamura began his karate training in 1953 at age eleven. His first experiences were in the Goju style under the instruction of Kei Miyagi Sensei, the son of the founder of the style. In 1956, Nakamura began studying with Masutatsu Oyama, the founder of Kyokushin Karate, and in 1959, he earned his shodan rank. At the time, he was the youngest Kyokushin student in Japan to receive a black belt. In 1961, at age nineteen, Nakamura debuted on the tournament scene with a first place triumph in the All-Japan Student Open Karate Championship. The following year, Nakamura became a Japanese national hero by knocking out a Thai kickboxing champion in a match to determine which nation had the superior martial art. Nakamura would win many more tournaments throughout his competition career. Around this time, he also began teaching karate to others.

He served as the chief instructor at Camp Zama, a U.S. military base near Tokyo, from 1961 to 1965 and coached the Toho Medical University karate team for 3 years. While earning his seventh dan in Kyokushin Karate, Nakamura also served as the chief instructor at the Kyokushin Karate Honbu in Tokyo. In 1966, Nakamura was personally selected by Masutatsu Oyama to help bring the true spirit of karate to America. That year, Nakamura moved to New York City and began teaching Kyokushin Karate at a small dojo in Brooklyn. In 1971, Nakamura established the North American Kyokushin Karate headquarters. He served as the American head of Kyokushin Karate for a decade and trained and developed many skilled students in that period. In 1976, Nakamura respectfully withdrew from Kyokushin Karate. The same year, he established the World Seido Karate Organization, which reflected his own beliefs about the true meaning of karate.