Under the guidance of renowned American trainer Eddie Townsend, Ishimatsu developed into a polished contender. His first world title opportunity came on June 6, 1970, when he challenged unified lightweight champion Ismael Laguna in Panama, losing by 13th-round technical knockout. Three years later, on September 8, 1973, he faced future Hall of Famer Roberto Durán for the WBA Lightweight Championship in Panama and was stopped in the 10th round.

Ishimatsu achieved boxing immortality on April 11, 1974, when he knocked out Mexico's Rodolfo "Gato" González in the eighth round at the Tokyo Metropolitan Gymnasium to capture the WBC Lightweight World Championship. The victory made him the first Japanese boxer in history to win a world lightweight title.

His championship reign became one of the defining periods in Japanese boxing. Ishimatsu successfully defended the WBC lightweight crown five times, including a 12th-round knockout victory in a rematch with González in November 1974 and a unanimous 15-round decision over former undisputed lightweight champion Ken Buchanan in February 1975. Despite trailing during portions of the fight, Ishimatsu rallied strongly in the later rounds to earn the verdict from all three judges.

After making five successful defenses, Ishimatsu lost the title to Puerto Rico's Esteban De Jesús by unanimous 15-round decision on May 30, 1976, in San Juan, Puerto Rico. The bout attracted enormous interest and reflected Ishimatsu's popularity throughout the boxing world.

Seeking another world championship, Ishimatsu moved up to junior welterweight and challenged WBC champion Saensak Muangsurin in 1977 but was stopped in six rounds. He retired following a non-title defeat on June 20, 1978, concluding a professional career that spanned more than a decade. His final record stood at 31 victories, including 17 knockouts, against 14 defeats and six draws in 51 professional bouts.

Outside the ring, Ishimatsu became an enduring cultural phenomenon in Japan. Although often portraying a comical "boke" character on television variety programs, he was respected for his sharp boxing analysis and insightful commentary. He became a fixture on Japanese television, appearing in quiz shows, dramas, films, and entertainment programs for decades.

Among his most lasting contributions to Japanese popular culture was the phrase "Guts Pose" (ガッツポーズ), inspired by his trademark post-fight celebration in which he pumped his fists in triumph. The expression remains widely used in Japan to describe a victory pose. His recurring catchphrase "OK Bokujo" ("OK Corral") became a national sensation and was recognized as one of Japan's most memorable pop-culture expressions.

Ishimatsu also enjoyed a successful acting career, appearing in international films including the movie Empire of the Sun and the movie Black Rain, along with numerous Japanese films and television productions. He often credited his desire to work alongside Japanese screen legend Ken Takakura as the motivation behind his transition into acting.

Beyond boxing and entertainment, Ishimatsu remained deeply connected to the sport. He served as a founding member and inaugural chief of the Boxing World Champions Club Japan, an organization composed of Japanese world champions dedicated to promoting boxing and supporting future generations of fighters. He also unsuccessfully ran for election to Japan's House of Representatives in 1996.

Guts Ishimatsu leaves behind his wife, three children, and two grandchildren. His remarkable journey—from an inexperienced teenager entering the professional ranks to a pioneering world champion and national celebrity—secured his place among the most influential figures in Japanese boxing history.

The global boxing community mourns the loss of a trailblazer whose achievements helped elevate Japanese boxing onto the world stage and whose charisma made him a beloved figure far beyond the sport.