Born in Tokyo, Sanada was scouted by an entertainer while playing with the son of the actor Kokichi Takada who lived in the same condominium. After working as a model for a magazine for young children, he joined the Himawari Theatre Group at the age of five. Originally planning to be an action movie star, he studied Shorinji Kempo and later took up Kyokushin kaikan karate. He began playing baseball at age 8 as a catcher. Sanada was mentored by actor Shinichi "Sonny" Chiba, and was a member of his Japan Action Club (JAC), becoming his protégé.
At the age of five, Sanada had his first role in the 1966 film Game of Chance (浪曲子守唄) as the son of Chiba's character. He had several roles in Japanese television and films, but quit temporarily so he could focus on living a normal childhood. He said this gave him "a good chance to watch movies as the audience", helping his perspective. By his teenage years he knew how to act, fight, sing, dance, perform stunts, ride horseback, and had developed versatility. Sanada had noticed that many popular actors would develop such skills, and felt it was "the best service for the audience" to do so himself before he practised acting again. He gained attention for his dangerous stunt of jumping from a helicopter in the 1979 film G.I. Samurai, action directed by Sonny Chiba. In 1981, he appeared in the smash-hit movie Samurai Reincarnation directed by Kinji Fukasaku, in which he played an iga-ryū ninja.
