Demura was born on September 15, 1938, in Yokohama, Japan. At the age of 9, he began training in karate and kendo under an instructor named Asano. At the age of 12 started training under Ryusho Sakagami in Itosu-kai karate. Demura received his 1st dan black belt in 1956, and won the East Japan Championships in 1957. In 1959, he began training in kobudo, a style of Okinawan weapons training, under the direction of Taira Shinken. In 1963, Demura met martial arts scholar Donn Draeger, who introduced him to Dan Ivan, who would eventually bring him to the United States of America as a karate instructor.

In 1965, Demura came to the United States, representing the Japan Karate-do Itosu-kai. From his base in southern California, he became well known for his karate and kobudo skills. In 1971, he was ranked 5th dan, and he remained at that rank until at least 1982. Through the 1970s and 1980s, Demura wrote several martial arts books, including: Shito-Ryu Karate (1971), Advanced nunchaku (1976, co-authored), Tonfa: Karate weapon of self-defense (1982), Nunchaku: Karate weapon of self-defense (1986), Bo: Karate weapon of self-defense (1987), and Sai: Karate weapon of self-defense (1987).

In the 1980s, Demura became involved in the Karate Kid series of films. He was the stunt double for Pat Morita, who played Mr. Miyagi. Demura has appeared in several films and documentaries, including: The Warrior within (1976), The Island of Dr. Moreau (1977), The Karate Kid (1984), The Karate Kid, Part III (1989), Shootfighter: Fight to the death (1992), Rising Sun (1993), The next Karate Kid (1994), Masters of the martial arts (1998, presented by Wesley Snipes), Mystic origins of the martial arts (1998), Modern warriors (2002), XMA: Xtreme Martial Arts (2003), and Ninja (2009).

In 1986, Demura was promoted to 7th dan in Shito-ry? karate. In 2001, he was expelled from the Itosu-kai, and became the Director of Shito-ry? Karate-do Genbu-kai. In 2005, he was promoted to 9th dan. He currently resides in Santa Ana, California.