Jimmy Miyaji was born in downtown Honolulu in 1928. A skinny and asthmatic child, he began his Martial Arts training in Judo around 1949. He studied with several sensei, including Richard Takamoto (son-in-law of Henry Seishiro Okazaki) at the Aiea Recreation Center, Yasuyuki Sakabe (on Maui), and Yukiso Yamamoto at the old Nuuanu YMCA.
Around 1955, Miyaji saw an advertisement for a new dojo opened by Masaichi Oshiro. This was the Te-Ken Jutsu Kai dojo at the McCully Community Center. Oshiro had been a student of William Chow at the Nuuanu YMCA. Chow, in turn, was a student of Masayoshi James Mitose, who popularized Kenpo Karate (or Kempo Jiu-Jitsu) in Hawaii. In 1959, Oshiro Sensei visited Japan for one year, where he studied Goju-Ryu under Gogen Yamaguchi, and Okinawa, where he studied under Yagi Meitoku.
In the late 1950s, Sensei Oshiro began converting his school to the Goju karate system. Senseis Bingo and Miyagi decided to start their own school and separated from Sensei Oshiro.
Miyaji then began to train with Richard Kim, whom he had met around 1957. Sensei Miyagi then separated from Sensei Bingo and started his own school under the Bu Toku Kai system which was headed by Professor Richard Kim of San Francisco, California. Kim was a merchant seaman, and visited Hawaii often on his way to and from the Orient. Miyaji trained with Kim for about the next 40 years and became one of his strongest supporters.
In 1961, Miyaji (along with Tommy Morita and Winfred Ho) co-sponsored the visit of Tsuyoshi Chitose, founder of Chito-Ryu and a student of several prominent instructors, including Seisho Aragaki, Kanryo Higashionna, Chomo Hanashiro, and Chotoku Kyan. In the 1960s, he trained with Hirokawa Kanazawa (for a few months), and in the 1970s he trained with Kenneth Murakami (for 3 or 4 years).
Because of the wide range of his experience, Miyaji practices and teaches over 50 Karate and Kobudo kata. Once, when he was visiting a neighboring island, he discussed Karate with some first generation Okinawans. They immediately recognized the Naihanchi Shodan kata when he performed it for them.
Miyaji was a founding member of the Hawaii Karate Congress and the Hawaii Karate Kodanshakai. He is one of only a few people in Hawaii to be a member of the Dai Nippon Bu Toku Kai. On November 13, 2004, he was inducted as a member of the Martial Art Hall of Fame of the International Division of the Dai Nippon Bu Toku Kai.
Miyaji has always kept the fees for his Karate and Kobudo classes at the Kenshukan dojo in Waipahu very low. He has taught innumerable children and adults, some of whom have gone on to teach their own classes.
James Miyaji's top students include, Rudy Castro (Shihan Dai), Dennis Sato, Ralph Sakauye, Nito Batalon, Eddie Santos, Shawna Carino, Darcie Mikami, Garin Miyaji (his grandson), and Clayton Gushiken.