Zenjiro Watanabe started skating at elementary school but took it seriously as a student from Keio University, at the time a center of figure skating in Japan. He took the title at the student championships in 1937, but never won a title at national championships in figure skating. Watanabe’s best results were second places in 1934 and 1937, and a fourth place in 1935, which earned him a place on the team for the 1936 Winter Olympics in Garmisch-Partenkirchen. On the expedition to Europe, Watanabe did not start in the European Figure Skating Championships, but was ranked 16th at the World Championships in Paris shortly after the Olympics.