Kammala wannan jumla: Jansen ya koma Pacific Coast League a matsayin mai horar da 'yan wasa tare da Seattle (1955 da 1957) da kuma Portland (1958 <unk> 60). Bayan kiran da tsohon abokin aikinsa Alvin Dark ya yi, Jansen ya koma Babban Wasannin a matsayin mai horar da 'yan wasa na San Francisco Giants a cikin, tare da Dark a matsayin manajan. Jansen ya kasance a matsayin mai horar da 'yan wasa na tsawon shekaru goma sha ɗaya, kuma ya taimaka wajen bunkasa Hall-of-Famers Juan Marichal da Gaylord Perry. A lokacin da yake aiki, Giants ya yi bayyanar a cikin 1962 World Series da
Gasar Gasar Gasar Kasa ta 1971. Daga nan sai ya koma aikin horar da MLB na karshe, yana kula da masu jefa kwallo ga Chicago Cubs a cikin <unk>73, yana aiki da tsohon manajan Giants, Leo Durocher, sannan kuma tsohon abokin aikin sa Whitey Lockman.