Ishii was born on June 10, 1953, in Uwajima, Ehime Prefecture, Japan. He is one of three siblings. As a boy, he was interested in sumo and baseball, and was also introduced to gymnastics. While Ishii was in junior high school, a book by Masatoshi Nakayama sparked his interest in Shotokan karate, but a Sonny Chiba film later inspired him to pursue training in Kyokushin karate.

Ishii began training in Kyokushin karate under Hideyuki Ashihara, who was then a senior instructor in Kyokushin’s International Karate Organization (IKO). By the time he was 16 years old, he had established a local Kyokushin dojo (training hall) under his instructor’s supervision. Six years later, in 1975, he opened a Kyokushin dojo in Osaka, and this was a very successful venture.

When Ashihara left the IKO a few years later, Ishii followed, but then left Ashihara’s organization after only a few months. He founded his own organization, Seidokaikan Karate, in 1980. Ishii’s organization had dojo in the Kansai region. Within the next two years, he was promoting televised full-contact karate tournaments. In 1983, Ishii became the first Chairman of the newly formed All Japan Budo Promotion Association. Seidokaikan’s reputation grew as Ishii’s students, such as Masaaki Satake, Toshiyuki Yanagisawa, and Toshiyuki Atokawa, earned tournament victories.

Following almost a decade of development, Ishii staged the inaugural K-1 tournament in Yoyogi Hall, Tokyo, in April 1993. According to The Japan Times, the “K” element came from kakutogi (a Japanese collective noun for combat techniques) and the “1” element came from the competition’s single weight division and the champion’s unique position (given the single weight division). According to Black Belt magazine, Ishii said that he chose the “K” element since it was the first letter in the names of karate, kickboxing, kung fu, kempo, and many other combative arts. K-1’s official website states that the “K” element also stands for “King.”

Over the next ten years, the K-1 competition expanded to 24 events each year, across Japan, Europe, and North America. In January 2003, Black Belt magazine named Ishii as its Man of the Year for 2002. Together with late K-1 fighter Andy Hug (1964–2000), Ishii supported the production of Street Fighter II: The Animated Movie (where he voiced the character of Fei-Long in the original Japanese audio) by creating fight scenes that utilized real-life combat techniques. In the PlayStation video game titled K-1 Revenge, he also appears as the fighting boss “Master Ishii”, whose character can be unlocked in the video game when completing the single player mode for the first time.