Born in Tokyo, Dezaki took an unconventional path into anime. While studying literature at Hosei University, he worked full-time at Toshiba, where he even coached the company's nine-member volleyball team. Throughout all this, he was quietly building connections and skills in film production, assembling his own anime production team on the side.

After seven years at Toshiba, he made the leap—resigning from his job and leaving university mid-term to pursue animation seriously. He joined Gisaburou Sugii's Art Fresh alongside his younger brother Osamu, both of them eventually becoming notable anime directors in their own right. Early credits include writing for Attack No. 1 and storyboarding Kyojin no Hoshi.

By 1969, Dezaki went freelance, handling scripting, production, and direction across major studios like Tokyo Movie Shinsha, Tatsunoko Production, and Sunrise. In 1977, he founded Magic Bus and directed Shin Kyojin no Hoshi, which became one of his signature works. He's best known for his prolific output and technical skill across multiple roles in anime production.

A minor note on his name: while some sources list his given name as "Satoshi," Dezaki himself has stated it should be read as "Tetsu."