Sonoda was born in Yanagawa, Fukuoka, Japan. He attended the Fukuoka Institute of Technology, and won a gold medal at the 1969 World Judo Championships along with his older brother, Yoshio, who won the gold medal in the lightweight (-63 kg) division. After graduating, he began work at the Maruzen Oil Company (current Cosmo Oil Company), where he continued to practice judo. He joined the Fukuoka prefectural police force in July 1972, at the invitation of the police force's judo instructor. Sonoda had competed in the All-Japan Judo Championships 10 times from age 19, but Shozo Fujii had won the competition for three consecutive years prior to 1976, when the championship served as the qualifier for the 1976 Summer Olympics, and was seen as a lock for Japan's Olympic judo team. However, Sonoda defeated Fujii by a very close decision, gained his first appearance at the Olympics at age 29. He defeated Valeriy Dvoinikov of the Soviet Union by "koka" from an Osoto Gari in the finals to capture the gold medal.